feat(foundry): merge-of-stems multimodel — Roformer vocals + demucs rest (#9)

PLN's "merge of stems": the best vocal isolation cascaded into the best 4-way split.
There is no 4-stem Roformer, so MultiModelBackend orchestrates a cascade —
  1. Roformer (audio-separator, py3.12 venv) isolates vocals + instrumental;
  2. demucs runs on the INSTRUMENTAL (vocals already gone → cleaner) for drums/bass/other;
  3. merge = Roformer vocals ⊕ demucs drums/bass/other in the standard 4-stem contract.

Refactored separate.py for composite backends: a backend declares what it `produces`
and (for composites) a `run_composite` hook; the subprocess runner is factored into
_run_cmd so the cascade can drive several sub-separations. RoformerBackend is now wired
(was a stub): audio-separator CLI, 2-stem (vocals/instrumental), normalize() globs the
"(Vocals)"/"(Instrumental)" output names. The default normalizer maps by produces[].

Verified end to end on the Loituma source: merge → vocals (Roformer) + drums/bass/other
(demucs-on-instrumental), 4 clean PCM_16/44.1k stems. All three backends report
available on this box. GUI renders whatever stems a backend emits (demucs/merge 4,
roformer 2 incl. instrumental), canonical roles first. 53 tests (added: registry has the
three engines, roformer produces 2 + builds its cmd, merge is composite/4-stem).

Keeps the 2-stem Roformer door open as its own backend (vocal isolation), and the merge
as the flagship. Foundry's engine registry now showcases the pluggable design PLN wants
to grow into a product/API/service.

tools/foundry/tests/test_engine.py

@@ -61,8 +61,21 @@ def test_catch_save_load(tmp_path):
 
 
 # ── backend registry ─────────────────────────────────────────────────────
-def test_registry_has_both_engines():
-    assert set(S.REGISTRY) == {"demucs", "roformer"}
+def test_registry_has_the_engines():
+    assert set(S.REGISTRY) == {"demucs", "roformer", "merge"}
+
+
+def test_roformer_produces_two_stems_and_builds_cmd():
+    be = S.get_backend("roformer")
+    assert be.produces == ("vocals", "instrumental")
+    cmd = be.build_cmd(Path("/w/s/source.webm"), Path("/tmp/o"), model="m.ckpt")
+    assert "--model_filename" in cmd and "m.ckpt" in cmd and "--output_dir" in cmd
+
+
+def test_merge_is_composite_producing_four_stems():
+    be = S.get_backend("merge")
+    assert be.composite is True
+    assert be.produces == ("drums", "bass", "other", "vocals")
 
 
 def test_demucs_cmd_build():
@@ -81,9 +94,11 @@ def test_demucs_stem_dir():
     assert d == Path("/w/s/_separated/htdemucs/source")
 
 
-def test_roformer_registered_but_unavailable():
-    ok, hint = S.get_backend("roformer").available()
-    assert ok is False and "engine2" in hint
+def test_backend_available_returns_status_and_hint():
+    # availability depends on which venvs exist on the box; the contract is (bool, str)
+    for name in ("demucs", "roformer", "merge"):
+        ok, hint = S.get_backend(name).available()
+        assert isinstance(ok, bool) and isinstance(hint, str) and hint
 
 
 def test_unknown_backend_raises():

tools/foundry/ui/index.html

@@ -166,7 +166,10 @@ function catchCard(c){
     `<div class="stem"><span class="tag" style="color:var(--mute)">original</span>
      <audio controls preload="metadata" src="/media/${c.slug}/${c.source_path}"></audio></div>`);
   if(c.separated){
-    STEMS.forEach(name=>{ const s=c.stems.find(x=>x.name===name); if(s) el.appendChild(stemRow(c.slug,s)); });
+    // render whatever stems this backend produced (demucs=4, roformer=2, merge=4),
+    // canonical roles first then any extras (e.g. roformer's instrumental)
+    const ord=s=>{const i=STEMS.indexOf(s.name);return i<0?99:i;};
+    [...c.stems].sort((a,b)=>ord(a)-ord(b)).forEach(s=>el.appendChild(stemRow(c.slug,s)));
     const lp=document.createElement("div"); lp.className="loops"; lp.id="lp-"+c.slug;
     lp.innerHTML=`<div class="loops-head"><span class="lbl">loops</span>
       <select class="mode" data-mode="${c.slug}" title="loop vs chop mode">