Sample Curator v0.1.8

v0.1.8 — Stem separation (vault-2nnt)

Auto-separate stems on new recordings, retroactive multi-select for
existing clips. Demucs runs in the runtime worker subprocess
(installable via Settings → Analysis & ML → Stems extra).

Slices:
- 81f7653: sidecar [stems] extra + ml_worker.separate_demucs handler
  (apply_model API, MPS device, model.eval); runtime-pyproject.toml
  build fix ([tool.uv] package = false)
- ca88f74: Rust stems.rs module + separate_stems Tauri command +
  sidecar separate_stems handler + JobType::StemSeparation +
  BatchActionsPanel "Separate stems" button with progress events
- c19623d: RecorderConfig.auto_stem_separation + Settings → Stems
  toggle + recorder_save_to_library auto-trigger + arm_session
  inherits global default at arm-on

Tags applied:
- parent:<source_id> + stem:<role> on each stem sample
- stems-done on the source (idempotency)

Acceptance criteria all met: install, non-blocking jobs, parent/stem
tagging, idempotency, duration threshold, global Settings toggle,
active-session toggle, multi-select retroactive path.

Tests: 118 sidecar, 511 frontend, 202 rust — all green.