Support half-degree setpoints driven by device metadata

[jeffscottmtl]

Closes #60, and covers the Eve Thermo case from #102.

What this does

Temperature setpoints (thermostat target, heater-cooler heating and cooling thresholds, and Home Assistant climate targets) now step and display in the increment the device actually advertises, typically 0.5 °C, instead of hardcoded whole degrees. A 21.5° setpoint set from Apple Home finally reads 21.5° rather than 22°, and the ± steppers can reach it.

It is metadata-driven, so it adapts per device: a 0.5° radiator steps by 0.5°, and a device that only reports whole degrees is completely unchanged.

In Celsius mode, a heat-mode setpoint of 21.5° now displays 21.5° (previously 22°), one + click snaps it to 22°, and an Auto-mode range of 20.5° / 24.5° shows both halves in the widened labels. Fahrenheit is deliberately untouched: the same 21.5 °C setpoint still reads 71° and still steps one displayed degree per click (the #137 behaviour). Happy to attach rendered screenshots if useful.

How it works

• Metadata through the bridge. ServiceData carries step/min/max for the target temperature and both thresholds, following the existing rotationSpeedMin/Max and colorTemperatureMin/Max precedent. On HomeKit they come from HMCharacteristicMetadata (stepValue/minimumValue/maximumValue); on Home Assistant from target_temp_step/min_temp/max_temp. HA values are normalized to internal Celsius, and because the step is a delta it scales by 5/9 without the freezing-point offset on Fahrenheit servers.
• Sane UI step. TemperatureFormatter.uiStep rounds the advertised step up to the nearest 0.5 °C. HomeKit's spec default step is 0.1 °C, which is unusable one click at a time, so 0.5 is the floor. Missing metadata falls back to 1.0, i.e. no behaviour change.
• Grid-snap stepping. In Celsius the value snaps to the step grid, so an off-grid 21.3 steps to 21.5 or 21.0. The Fahrenheit branch of step is untouched.
• Honest display. A new formatSetpoint shows one decimal only when the value is fractional (21° vs 21.5°); the existing whole-degree format and its ceil are left exactly as they are, so sensor readouts and Fahrenheit are unaffected.
• Metadata-driven clamps. Setpoint clamps use the device min/max when present, with the previous constants (10/30, 16/30) as fallback. The 1 °C heat/cool spread is kept, since ecobee rejects writes with a smaller gap.
• Bonus fix. ACMenuItem stepped the raw Celsius value by 1 (1.8 °F per click), the same class of bug as [Bug] Changing Temperature Skips from 72 to 70 #137 but never fixed for heater-coolers. It now goes through TemperatureFormatter.step like the other climate items, so Fahrenheit AC stepping moves one displayed degree.
• Layout. Auto-mode range labels widen to fit half-degree strings; mode buttons narrow slightly to keep the row inside its width budget.

Tests

The suite goes from 987 to 1023 (36 added), all green on xcodebuild test -scheme Itsyhome -destination "platform=macOS". Coverage includes the uiStep derivation table, Celsius grid-stepping and float-noise cases, formatSetpoint whole/half/negative, HA unit scaling, ServiceData codable round trips and legacy-JSON decode, and real label-text assertions through the menu-item views. The AC Fahrenheit fix and the new threshold-clamp behaviour are pinned with mutation-verified regression tests (reverting the fix makes them fail).

The three existing #137 Fahrenheit regression tests are byte-identical and still pass.

Notes

• Half degrees are a Celsius-mode feature, matching the [Feature request] add support for 0.5 degree setting for heating #60 request. Fahrenheit users keep whole-degree stepping and display by design.
• HAClimateMenuItem intentionally keeps its trailing margin, so its Auto layout differs slightly from the HomeKit items; noted in a comment.
• One pre-existing adjacent bug spotted but left untouched to keep the diff focused: HA scene target temperatures skip unit normalization (EntityMapper generateSceneActions), so on Fahrenheit servers the scene value is in °F while everything else is Celsius. Happy to send that as a separate PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_018RGy77ui93iHqVnNLtrVWF