Negative-prompt and guidance steering for generation (incl. cleaner stem-ish layers)

[brxs]

Depends on #54 (negative-prompt + cfg/apg).

Problem

Steering generation today is prompt-only and additive — you can ask for things but not against them, and you cannot trade prompt-adherence for diversity. That makes layerable material hard: a "bassline" prompt still tends to come back with drums, muddying a layer under a deck (the SA3 cousin of the drum-sit problem, #50). #54 exposes --negative-prompt + --cfg/--apg, which give exactly these controls.

Proposed approach

Expose two steering controls on the generate surface: "avoid" (a negative prompt, with a few one-tap chips — no vocals / no drums / no cymbals / no melody) and an adherence ↔ diversity knob mapped to --cfg (with --apg where it helps). Together these make cleaner stem-ish layers ("bassline, no drums, no melody") for layering under a deck, and tame or loosen prompt-following. Persist the chosen avoid/guidance with the recipe (ADR-0022).

Acceptance criteria

• A negative prompt audibly suppresses the named element (a "no drums" bassline comes back materially drum-lighter than without).
• The adherence/diversity knob visibly trades prompt-fidelity for variety; defaults reproduce today's behavior (cfg unchanged from now).
• Avoid/guidance settings save with the pad recipe and reload.
• Backend validates cfg/apg bounds (from Thread Stable Audio 3's full generation surface (audio-to-audio, inpainting, CFG, seed) through the pipeline #54); strings via i18n; house style; npx tsc -p tsconfig.app.json --noEmit clean.

Scope & non-goals

In scope: negative-prompt + guidance controls and the stem-ish layering use. Non-goals: the plumbing (#54); true stem separation (this is prompt-conditioned, not a separator); the palette builder (separate issue); drum-sit on the Magenta deck (#50).