Gauge redline zones + C/F units toggle + cleaner dials

Gauges:
- Optional per-metric warning zones (warn_hi/redline_hi/warn_lo/redline_lo) in
  the profile schema; gauges draw colored redline/warn bands and color the
  needle + readout by zone. Default neutral when unset (no false redline).
- Removed the value progress-arc fill (it dominated the dial / looked wrong) ->
  clean tach face: bezel, ticks, numeric scale, needle, redline band, readout.
- Auto-derivation rejected: bad direction/threshold vary per metric, so zones
  are config (with a sensible neutral default).

Units:
- New Units menu: Temperature C / F. Converts gauges, graph, table, and PID
  browser (values, scale, zones, unit labels) at display time; data stays C.

Ford 6.0 profile: zones for ICP (red<500), FICM (red<40/amber40-48/green48+),
ECT/EOT (high redline), RPM (redline 3800), boost, battery; tightened FICM
(38-52) and battery (9-15) ranges so redline bands land sensibly.

Docs: profiles/PROFILE_SPEC.md -- canonical, AI-agent-ready profile spec
(schema, formula language, zones, confidence, rules); README points to it.

Validated headless: zones parse/classify, F conversion (112C->233.6F, zones
converted), gauges render; obdcore + diagnostics tests pass.

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

obdcore/registry.py

@@ -26,6 +26,26 @@ class Pid:
     round: int = None               # display rounding (None=raw float, 0=int)
     deps: Tuple[str, ...] = ()
     notes: str = ""
+    # optional gauge warning zones (all None = neutral, no redline drawn).
+    # high-side: value >= threshold -> warn/redline.  low-side: value <= -> warn/redline.
+    warn_hi: float = None
+    redline_hi: float = None
+    warn_lo: float = None
+    redline_lo: float = None
+
+    def zone(self, v):
+        """Classify a value as 'crit' (red), 'warn' (amber), or 'ok' (green)."""
+        if v is None:
+            return "ok"
+        if self.redline_hi is not None and v >= self.redline_hi:
+            return "crit"
+        if self.redline_lo is not None and v <= self.redline_lo:
+            return "crit"
+        if self.warn_hi is not None and v >= self.warn_hi:
+            return "warn"
+        if self.warn_lo is not None and v <= self.warn_lo:
+            return "warn"
+        return "ok"
 
 
 @dataclass