feat(f051): build flag to compile out sine startup for flash savings (~1 KB+)

[dakejahl]

Summary

Sine startup is the single largest chunk of flash we could reclaim on the F051, but ARK boards use it, so we're keeping it enabled by default and tracking a build-flag opt-out here.

Context

Enabling LTO (#1) bought cross-TU inlining on the F051 hot path at a cost of ~2.7 KB flash. The worst-case ARK_4IN1_F051 now sits at ~92.6% of the 27 KB app region (~2 KB headroom), and we want more room for the active-phase demag work (#2).

Sine startup (use_sine_start) is a runtime setting whose code is always linked in regardless of configuration:

• pwmSin[] table — 720 B
• advanceincrement() — 344 B
• plus scattered conditionals in the 20 kHz interrupt and the main loop

That's ~1 KB+ of flash. Removing the hot-loop branches would also hand a few cycles back to the commutation path, which directly helps the demag effort.

Proposal

Gate sine startup behind a target #define (e.g. DISABLE_SINE_START), defaulting to enabled so existing boards are unaffected, and turn the opt-out on only for flash-tight targets that don't need it. Measure the exact saving when implemented.

Related deferred flash levers

• Audit the remaining signed divisions and make them unsigned to drop __divsi3 (~460 B).
• BlueJay custom-startup-tune engine gated off on ARK F051 (~424 B) — handled alongside the F051 perf work, not part of this issue.