feat(tide-table): foundry_stems — bridge to the Foundry demucs engine

Stem-aware emotion needs the voice isolated: the full mix is muddy for CLAP, but
the VOCALS carry valence, DRUMS+BASS carry arousal, and vocal presence is itself a
section/structure cue. ensure_stems(file, slug) consumes the Foundry's in-process
Python API (tools/foundry, the shared separation engine) → {mix,vocals,bass,drums,
other} absolute paths, idempotent via catch.json (re-runs free). Validated: 4-stem
demucs separation of a 6-min track (Bohemian Rhapsody) lands all four stems clean.
Feeds emotion_timeline's stem-aware analysis (#89) and the hexa lane (#90).

armada/tide-table/foundry_stems.py

+#!/usr/bin/env python3
+"""foundry_stems — thin bridge to the Foundry demucs engine for stem-aware emotion.
+
+The Foundry (tools/foundry) is the shared stem-separation engine; we consume its
+in-process Python API to turn a track into vocals/bass/drums/other, idempotently
+(catch.json caches the result, so re-runs are free). Why stems: the full mix is
+muddy for CLAP — the VOCALS carry valence, DRUMS+BASS carry arousal, and vocal
+presence is itself a section/structure cue. See ../../tools/foundry/README.md
+("Use as an API") and [[project_foundry]] / [[project_hexa_emotion_convergence]].
+
+    python3 foundry_stems.py <audio-file> [slug]      # separate one file, print stem paths
+"""
+import sys
+from pathlib import Path
+
+HERE = Path(__file__).resolve().parent
+FOUNDRY = HERE.parent.parent / "tools" / "foundry"
+WS = HERE / "_corpus_cache" / "foundry"
+STEM_NAMES = ("vocals", "bass", "drums", "other")
+
+
+def ensure_stems(audio_path, slug, *, shifts=1, log=print):
+    """Local audio file → {mix, vocals, bass, drums, other} → absolute Paths.
+    Reuses existing stems if the catch is already separated (idempotent)."""
+    import shutil
+    if str(FOUNDRY) not in sys.path:
+        sys.path.insert(0, str(FOUNDRY))
+    from engine import separate
+    from engine.model import Catch
+    WS.mkdir(parents=True, exist_ok=True)
+    d = WS / slug
+    if (d / "catch.json").exists():
+        c = Catch.load(d)
+    else:
+        d.mkdir(parents=True, exist_ok=True)
+        src = d / ("source" + Path(audio_path).suffix.lower())
+        if not src.exists():
+            shutil.copy(audio_path, src)
+        c = Catch(slug=slug, title=slug, source_path=src.name)
+        c.save(WS)
+    if not c.separated:
+        c = separate.separate(c, WS, backend="demucs", shifts=shifts, log=log)
+        c.save(WS)
+    cd = c.dir(WS)
+    out = {"mix": cd / c.source_path}
+    for s in c.stems:
+        out[s.name] = cd / s.path
+    return out
+
+
+def main():
+    if len(sys.argv) < 2:
+        sys.exit("usage: foundry_stems.py <audio-file> [slug]")
+    p = Path(sys.argv[1])
+    slug = sys.argv[2] if len(sys.argv) > 2 else p.stem
+    stems = ensure_stems(p, slug)
+    for name, path in stems.items():
+        print(f"  {name:<7} {path}")
+
+
+if __name__ == "__main__":
+    main()