Document cross-platform support (Windows/macOS/Linux)

The stack is portable by construction: PySide6/pyqtgraph/numpy/pyserial all
ship wheels for all three OSes (incl. Apple Silicon); obdcore has no
OS-specific code; the terminal dashboard's only platform code is guarded
(os.name=='nt' vs termios for POSIX = macOS+Linux).

- ARCHITECTURE.md: Cross-platform section -- portability rules (list_ports
  only, pathlib, no shelling out, platformdirs for config), the three per-OS
  seams (CH340 driver, PyInstaller per-OS packaging, Gatekeeper/SmartScreen).
- README: Setup now covers Windows (CH341SER), macOS (CH34xVCPDriver), Linux
  (in-kernel ch341 + dialout group) instead of Windows-only.

No code changes; obdcore tests still pass.

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

ARCHITECTURE.md

@@ -113,6 +113,43 @@ sensors (e.g. a serial/analog EGT/oil-PSI module) can feed extra channels later.
 - **PID discovery scan** in GUI (the brute-scan, auto-add hits).
 - Units toggle, night theme, big-touch cab mode.
 
+## Cross-platform support
+
+Target: **Windows, macOS, Linux** from one codebase. The whole stack is
+portable — `PySide6`, `pyqtgraph`, `numpy`, `pyserial` all ship wheels for
+all three (incl. Apple Silicon). `obdcore` is pure Python/IO with no
+OS-specific calls; the only platform code is the terminal dashboard's
+ANSI/key handling, guarded `os.name == "nt"` (Windows) vs `termios` (POSIX =
+macOS + Linux). Rules to keep it that way:
+
+- **Serial enumeration** goes through `serial.tools.list_ports` only — never
+  assume `COMx`. Port names differ per OS (`COM5` / `/dev/cu.usbserial-*` /
+  `/dev/ttyUSB0`); the GUI shows a dropdown from `find_ports()`, no typing.
+- **Paths**: `pathlib`/caller-supplied only; no hardcoded separators or drives.
+- **No shelling out** to OS tools in core; no `os.system`. (The Windows VT
+  enable via `ctypes.windll` is the one exception, fully guarded.)
+- **Config/recordings** live under a per-OS app-data dir
+  (`platformdirs`) rather than next to the script.
+
+Three seams that are inherently per-OS (not code we can unify):
+
+1. **CH340 USB driver** (the adapter, not our app):
+   - **Windows** — install WCH `CH341SER`.
+   - **macOS** — install the WCH `CH34xVCPDriver` (Mac App Store / WCH);
+     recent macOS bundles a CH34x driver but clones often need WCH's.
+     Port shows as `/dev/cu.wchusbserial*` or `/dev/cu.usbserial*`.
+   - **Linux** — `ch341` is in the kernel; zero install. Just add the user to
+     the `dialout` group for `/dev/ttyUSB0` access.
+2. **Packaging** — PyInstaller is per-OS (no cross-compile): build the `.exe`
+   on Windows, the `.app`/`.dmg` on macOS, an ELF/AppImage on Linux. We have
+   all three hosts available, or do it in CI. (Briefcase is the alternative if
+   we want macOS notarization/signing handled.)
+3. **macOS Gatekeeper / Windows SmartScreen** — unsigned builds warn on first
+   run; optional code-signing/notarization later (P5).
+
+Primary dev/use target is the Windows laptop; macOS/Linux are first-class for
+development (build the GUI against `MockLink` on any box) and supported targets.
+
 ## Roadmap
 
 - **P0 — core (this commit):** `obdcore` package + tests + this doc. *Next:*
@@ -130,5 +167,8 @@ sensors (e.g. a serial/analog EGT/oil-PSI module) can feed extra channels later.
 ## Dependencies
 
 - Runtime (core): `pyserial`.
-- GUI: `PySide6`, `pyqtgraph`, `numpy`.
+- GUI: `PySide6`, `pyqtgraph`, `numpy`, `platformdirs` (per-OS config/recording dirs).
 - Dev: `pytest`, `pyinstaller`.
+
+All are cross-platform with wheels for Windows / macOS (incl. Apple Silicon) /
+Linux, so `pip install` is the same everywhere.