perf: cut audio-CPU on the voice path + LTO flash savings

[joak0068]

No audible change; verified to build clean (FLASH 98.2% -> 97.1%).

What this changes

• Voice filter coefficients (Svf sinf+powf / Moog polynomial) are recomputed only when cutoff/resonance/filter-type change, not every sample -- a real saving for static-filter patches (cutoff held, no filter envelope).
• Unison detune frequency multipliers + 1/u gain are precomputed per block instead of a per-sample division per oscillator.
• The final hard clamp to [-1,1] is folded into the master limiter, dropping a whole extra per-block buffer pass in main.cpp.
• master.h: the param-smoothing coefficient uses params::audio::kBlockSize instead of a hardcoded 48, so block size is a clean one-line lever.
• Enable -flto (compile + link); keep usb_identity.c out of LTO so its descriptor override stays deterministic. Nets ~1.5 KB of internal flash back.

Note: the original "hoist SetFreq out of the loop" idea was dropped -- DaisySP's SetFreq is the costly call, not the oscillator's, so the coefficient guard above is where the saving actually is.

Checklist

• New tunable values live in src/config/params.h
• If a control changed, updated docs/MIDI_PROTOCOL.md and the params::midi map
• Tested on hardware (Daisy Seed + Hothouse) — describe below, or note if untested