Variations and reproducible seeds: 're-roll' and 'more like this' for generated pads

[brxs]

Depends on #54 (seed exposure + low-noise audio-to-audio).

Problem

Generation today is a one-shot: a pad comes back, and if it is almost right there is no way to nudge it — re-running the same prompt rolls a fresh random seed (sa3_mlx.py picks one when --seed is omitted) and the near-miss is lost. There is also no reproducibility: a great pad cannot be re-derived or tweaked because its seed was never captured. #54 exposes --seed and low --init-noise-level audio-to-audio, which together give the two operations performers expect: re-roll (same prompt, new seed) and vary (a small perturbation of this clip).

Proposed approach

• Capture the seed. sa3.generate() returns/records the seed it used; the samples-library recipe (ADR-0022, useDeck.ts:1339) stores prompt + seed + seconds + kind + steps so any saved pad is reproducible.
• Re-roll a slot: regenerate the same prompt with a fresh seed.
• Vary ("more like this"): feed the existing clip back as Thread Stable Audio 3's full generation surface (audio-to-audio, inpainting, CFG, seed) through the pipeline #54's init_audio at a low init_noise_level for a near-neighbor; a small "amount" knob spans subtle → loose.
• Variations land as new slots/library entries (non-destructive), each with its own recipe.

Acceptance criteria

• Every generated pad records the seed actually used; re-running a saved recipe (same prompt + seed + seconds + steps) reproduces the audio.
• A vary action produces an audible near-neighbor of the source clip; a re-roll produces a fresh take of the same prompt; both are non-destructive (new slot/entry) and carry their own recipe.
• Backend tests cover the seed round-trip (same seed ⇒ same bytes) and that vary differs from the source but less than a re-roll.
• Strings via i18n; house style; npx tsc -p tsconfig.app.json --noEmit clean.

Scope & non-goals

In scope: seed capture/replay, re-roll, vary. Non-goals: the plumbing (#54); reskinning external audio (that is the reskin issue — this varies a clip that is already SA3 output); a full prompt-history/branching UI.