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

profiles/README.md

@@ -7,6 +7,11 @@ scaling formulas), DTC meanings, and named presets. Load one in the app via
 
 **Contributions welcome** — add a profile for your vehicle and open a PR.
 
+> 📋 **The canonical, AI-agent-ready format is [`PROFILE_SPEC.md`](PROFILE_SPEC.md).**
+> Researching a new vehicle? Paste that spec into your AI agent and say *"produce
+> an OBDash vehicle profile JSON for <year make model engine> per this spec."*
+> The summary below is a quick reference; the spec is authoritative.
+
 ## Current profiles
 
 | File | Vehicle | Notes |