Phase 1-2 (PLAN_MASTERBUS_FX_RACK_VST): tests cho render_fx_chain + API fx-render; plan HOÀN THÀNH Phase 0-4

- test_native_render: render_fx_chain builtin gain e2e (bridge --render-fx)
- test_plugin_api: POST /api/v1/plugins/fx-render builtin gain
- PLAN_MASTERBUS_FX_RACK_VST.md: cap nhat trang thai cac phase (0-4 da implement tu cac commit truoc + phan nay)

tests/test_native_render.py

@@ -66,3 +66,30 @@ def test_plugins_bridge_branch_dispatch(monkeypatch):
     with pytest.raises(HTTPException) as ei:
         pl._render_midi_notes_pedalboard("__no_such__", [], bpm=120.0, sample_rate=44100)
     assert ei.value.status_code == 404
+def test_render_fx_chain_builtin_gain_e2e():
+    """render_fx_chain (PLAN_MASTERBUS_FX_RACK_VST Phase 1): builtin gain
+    +6dB qua bridge --render-fx, không cần VST. Output tồn tại, đúng sr,
+    RMS gấp đôi input."""
+    import math
+    import numpy as np
+    import soundfile as sf
+    exe = native_render.find_bridge_exe()
+    if not exe:
+        pytest.skip("bridge exe missing")
+    sr = 44100
+    t = np.linspace(0, 1.0, sr, endpoint=False)
+    data = (0.1 * np.sin(2 * np.pi * 440 * t)).astype(np.float32)
+    with tempfile.TemporaryDirectory() as td:
+        inp = os.path.join(td, "in.wav")
+        sf.write(inp, data, sr, subtype="FLOAT")
+        out = native_render.render_fx_chain(
+            input_wav=inp,
+            fx_chain=[{"type": "builtin", "id": "gain", "params": {"db": 6.0}}],
+            sample_rate=sr,
+        )
+        assert os.path.isfile(out)
+        d, _ = sf.read(out, dtype="float32", always_2d=True)
+        rms = float(np.sqrt(np.mean(d ** 2)))
+        assert rms > 0.09  # ~2x 0.05
+        info = sf.info(out)
+        assert info.samplerate == sr