Derive speaker formant override keys from declared formant bank

[ctoth]

Problem

Speaker formant override projection still hardcodes F1 through F10 in the frontend, even though control-score projection was made dynamic. This leaves one remaining fixed-formant-count assumption in the FE signal path.

Evidence

• src/tts-frontend.ts defines SPEAKER_FORMANT_KEYS = ["F1" ... "F10"].
• src/tts-frontend.ts iterates that list when applying speaker profile formant overrides.
• src/control-score.ts no longer has the old index <= 10 loop, so the frontend override path is now the remaining mismatch.

Acceptance criteria

• Derive speaker formant override keys from the active formant-bank/graph registry or another declarative synthesizer capability surface.
• Remove SPEAKER_FORMANT_KEYS as a fixed F1-F10 list.
• Add a focused test proving an F11 speaker override is carried when the active synthesizer declares that formant.
• Do not reintroduce a fixed upper bound in TypeScript.
• npm.cmd run typecheck:core passes.