Add Tools/Diagnostics: thread-safe DTC read/clear + Diagnostics panel

The polling thread owns the ELM327, so reading/clearing trouble codes from
the GUI thread would race PID reads and corrupt the stream. Add a one-off
command path that serializes ad-hoc link work onto the polling thread.

obdcore/scheduler.py:
- PollScheduler.run_oneoff(fn, timeout) enqueues a callable (queue.Queue +
  threading.Event) and blocks for its result, re-raising the callable's
  exception. tick() drains queued one-offs at its very top, so they run on
  the same thread that does PID reads -- never concurrently. When the
  scheduler thread isn't running, the job is drained inline on the caller
  (still serialized vs tick(), safe because nothing else touches the link).

gui/controller.py:
- Controller.read_dtcs() -> {"stored","pending","permanent"} (modes 03/07/0A,
  svc 0x43/0x47/0x4A) and clear_dtcs() -> bool. Both route through the
  scheduler one-off when a scheduler exists, else call the link directly.

gui/main.py:
- Diagnostics menu (Read Codes / Clear Codes...) and a right-side QDockWidget
  listing codes grouped Stored/Pending/Permanent. Each row is code +
  description + system from DtcDatabase; no_start codes are flagged bold red.
- Clear is guarded by a confirmation warning (erases codes + freeze frame;
  honest "the code comes right back" / permanent-codes-won't-clear tone from
  run_clear in obd_reader.py). On confirm: clear, then re-read immediately and
  show whatever returned, reporting active faults that came straight back.

tests/test_diagnostics.py:
- one-off returns its value, re-raises exceptions, is drained before a tick's
  PID reads, and runs on a live background thread while polling continues.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

gui/main.py

@@ -45,6 +45,7 @@ class MainWindow(QtWidgets.QMainWindow):
         self._build_menubar()
         self._build_connection_bar()
         self._build_pid_browser()
+        self._build_diag_dock()
         self._build_center()
         self._build_statusbar()
         self._refresh_title()
@@ -73,6 +74,12 @@ class MainWindow(QtWidgets.QMainWindow):
         self.profm = mb.addMenu("&Profile")
         self._rebuild_profile_menu()
 
+        diagm = mb.addMenu("&Diagnostics")
+        self.read_dtc_act = self._act(diagm, "Read Codes", self._read_codes,
+                                      "Read stored / pending / permanent trouble codes")
+        self.clear_dtc_act = self._act(diagm, "Clear Codes…", self._clear_codes,
+                                       "Erase stored codes + freeze frame (mode 04)")
+
         viewm = mb.addMenu("&View")
         self.view_graph = self._act(viewm, "Graph View", lambda: self._set_view(0),
                                     checkable=True)
@@ -85,6 +92,9 @@ class MainWindow(QtWidgets.QMainWindow):
         self.show_pids = self._act(viewm, "Show PID Panel", self._toggle_pid_dock,
                                    checkable=True)
         self.show_pids.setChecked(True)
+        self.show_diag = self._act(viewm, "Show Diagnostics Panel", self._toggle_diag_dock,
+                                   checkable=True)
+        self.show_diag.setChecked(True)
         self.norm_act = self._act(viewm, "Normalize Graph (% of range)",
                                   self._sync_norm_from_menu, checkable=True)
         viewm.addSeparator()
@@ -202,6 +212,142 @@ class MainWindow(QtWidgets.QMainWindow):
         self.tree.resizeColumnToContents(0)
         self.tree.blockSignals(False)
 
+    # ---------- diagnostics dock (DTCs) ----------
+    def _build_diag_dock(self):
+        self.diag_dock = QtWidgets.QDockWidget("Diagnostics", self)
+        wrap = QtWidgets.QWidget()
+        lay = QtWidgets.QVBoxLayout(wrap)
+        lay.setContentsMargins(4, 4, 4, 4)
+        bar = QtWidgets.QHBoxLayout()
+        self.diag_read_btn = QtWidgets.QPushButton("Read Codes")
+        self.diag_read_btn.clicked.connect(self._read_codes)
+        self.diag_clear_btn = QtWidgets.QPushButton("Clear Codes…")
+        self.diag_clear_btn.clicked.connect(self._clear_codes)
+        bar.addWidget(self.diag_read_btn)
+        bar.addWidget(self.diag_clear_btn)
+        bar.addStretch(1)
+        lay.addLayout(bar)
+        self.diag_tree = QtWidgets.QTreeWidget()
+        self.diag_tree.setColumnCount(3)
+        self.diag_tree.setHeaderLabels(["Code", "Description", "System"])
+        self.diag_tree.setRootIsDecorated(True)
+        self.diag_tree.header().setStretchLastSection(False)
+        self.diag_tree.header().setSectionResizeMode(
+            1, QtWidgets.QHeaderView.Stretch)
+        lay.addWidget(self.diag_tree)
+        self.diag_hint = QtWidgets.QLabel(
+            "Connect, then Read Codes.  Bold red = no-start / drive-disabling.")
+        self.diag_hint.setWordWrap(True)
+        lay.addWidget(self.diag_hint)
+        self.diag_dock.setWidget(wrap)
+        self.diag_dock.visibilityChanged.connect(
+            lambda vis: self.show_diag.setChecked(vis))
+        self.addDockWidget(QtCore.Qt.RightDockWidgetArea, self.diag_dock)
+
+    _DIAG_GROUPS = [("stored", "Stored (mode 03)"),
+                    ("pending", "Pending (mode 07)"),
+                    ("permanent", "Permanent (mode 0A)")]
+
+    def _populate_diag(self, codes):
+        """codes: {'stored':[...], 'pending':[...], 'permanent':[...]}"""
+        self.diag_tree.clear()
+        total = 0
+        for bucket, label in self._DIAG_GROUPS:
+            lst = codes.get(bucket, [])
+            total += len(lst)
+            top = QtWidgets.QTreeWidgetItem([f"{label}  ({len(lst)})", "", ""])
+            f = top.font(0); f.setBold(True); top.setFont(0, f)
+            top.setFirstColumnSpanned(False)
+            self.diag_tree.addTopLevelItem(top)
+            if not lst:
+                none = QtWidgets.QTreeWidgetItem(["—", "(no codes)", ""])
+                none.setForeground(1, QtGui.QBrush(QtGui.QColor("#888")))
+                top.addChild(none)
+            for code in lst:
+                d = self.ctl.dtcdb.get(code)
+                it = QtWidgets.QTreeWidgetItem([code, d.desc, d.system])
+                it.setData(0, QtCore.Qt.UserRole, code)
+                if getattr(d, "no_start", False):
+                    red = QtGui.QBrush(QtGui.QColor("#e6194B"))
+                    bf = it.font(0); bf.setBold(True)
+                    for c in range(3):
+                        it.setFont(c, bf)
+                        it.setForeground(c, red)
+                    it.setToolTip(0, "No-start / drive-disabling fault")
+                top.addChild(it)
+            top.setExpanded(True)
+        self.diag_tree.resizeColumnToContents(0)
+        self.diag_tree.resizeColumnToContents(2)
+        return total
+
+    def _read_codes(self):
+        if not self.ctl.connected:
+            QtWidgets.QMessageBox.information(
+                self, "Not connected", "Connect (or tick Mock) before reading codes.")
+            return
+        try:
+            codes = self.ctl.read_dtcs()
+        except Exception as e:
+            QtWidgets.QMessageBox.critical(self, "Read failed", str(e))
+            return
+        total = self._populate_diag(codes)
+        self.diag_dock.setVisible(True)
+        self.show_diag.setChecked(True)
+        self.status.showMessage(
+            f"Read codes: {total} found "
+            f"(stored {len(codes.get('stored', []))}, "
+            f"pending {len(codes.get('pending', []))}, "
+            f"permanent {len(codes.get('permanent', []))})."
+            if total else "Read codes: none stored.")
+
+    def _clear_codes(self):
+        if not self.ctl.connected:
+            QtWidgets.QMessageBox.information(
+                self, "Not connected", "Connect (or tick Mock) before clearing codes.")
+            return
+        btn = QtWidgets.QMessageBox.warning(
+            self, "Clear codes?",
+            "This erases stored + pending codes AND freeze-frame data, and "
+            "resets emissions monitors.\n\n"
+            "Write the codes down first — and read them on a no-start before "
+            "clearing.  If the fault is still present the code comes right back.\n"
+            "Permanent codes (mode 0A) will NOT clear until the fault is fixed "
+            "and the vehicle self-clears them over several drive cycles.\n\n"
+            "Clear codes now?",
+            QtWidgets.QMessageBox.Yes | QtWidgets.QMessageBox.No,
+            QtWidgets.QMessageBox.No)
+        if btn != QtWidgets.QMessageBox.Yes:
+            self.status.showMessage("Cancelled. No codes cleared.")
+            return
+        try:
+            ok = self.ctl.clear_dtcs()
+        except Exception as e:
+            QtWidgets.QMessageBox.critical(self, "Clear failed", str(e))
+            return
+        if not ok:
+            QtWidgets.QMessageBox.warning(
+                self, "No acknowledgement",
+                "The ECU did not acknowledge the clear.\n"
+                "Make sure the key is in RUN and the vehicle is connected, then "
+                "try again.")
+            self.status.showMessage("Clear not acknowledged by ECU.")
+            return
+        # Re-read immediately so anything that came straight back is shown.
+        try:
+            codes = self.ctl.read_dtcs()
+        except Exception:
+            codes = {}
+        returned = self._populate_diag(codes)
+        if returned:
+            self.status.showMessage(
+                f"Cleared — but {returned} code(s) returned immediately "
+                "(active fault present).")
+        else:
+            self.status.showMessage("Cleared. No codes on re-read.")
+
+    def _toggle_diag_dock(self):
+        self.diag_dock.setVisible(self.show_diag.isChecked())
+
     # ---------- center (graph + table stack) ----------
     def _build_center(self):
         self.stack = QtWidgets.QStackedWidget()