Instrument identity is per-song, not per-bar: pin the sampled pick to the song seed

The live walk reseeds every bar (seed + serial*7919) for PATTERN variety, but that churned seed also fed the sampled-instrument pick — so every voice was silently swapping instruments EVERY BAR (pad: french_horns → synth_strings → ohh_voices → church_organ, bar to bar).

• The walk now stashes the pre-churn song seed as state.instrumentSeed (live.js makeWalk + the explorer's barVoiceEvents mirror) and forceSampled picks with it — the band stays the band for the whole song

Verified: engine, all-sampled, segment-parity byte-equal, wavout-seam, matrix 93/93, validate, resilience, audit, wavout (all routes), live, genre-viz.

▶ Play it: Highway Heat Death — where the synthesis is the identity, not the decoration https://stellate.app/?genre=synthwave&seed=4113&path=859.9909,878.9942,840.9942&xdur=13.122

9ff93bc https://github.com/aboard-io/stellate/commit/9ff93bc9c25352ede18bd622ccc308f595de7213