fix(tests): isolate plugin routes modules + redact .feedpak filenames (#736)

Two pre-existing failures the segfault had been masking (the run aborted at ~25%, so they never ran until #735 let the suite complete):

1) Tuner group (~24): plugins ship a bare-named routes.py, so sys.modules['routes'] leaked between plugin test dirs (achievements ran first, tuner got its module). Each plugin conftest now pops the stale 'routes' and an autouse fixture binds sys.modules['routes'] to that plugin's module for the duration of its tests (covers runtime 'import routes' in test bodies).

2) Diagnostics group (5): _SONG_FILENAME_RE never matched the tests' .feedpak/.archive filenames — it also lacked 'feedpak' (the current primary format), a real redaction gap. Added feedpak to the regex and switched the tests off the fake .archive to the real .feedpak. Verified: full suite 2183 passed, 0 failed.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/plugins/tuner/conftest.py

@@ -5,6 +5,8 @@ sys.path.insert(0, str(Path(__file__).parent.parent.parent.parent / 'plugins' /
 import pytest
 from fastapi import FastAPI
 from fastapi.testclient import TestClient
+# Drop a sibling 'routes' cached by another plugin's tests (bare-name collision).
+sys.modules.pop('routes', None)
 import routes as tuner_routes
 
 
@@ -22,3 +24,19 @@ def client(config_dir):
         "unregister_tuning_provider": lambda pid: None,
     })
     return TestClient(app)
+
+
+@pytest.fixture(autouse=True)
+def _bind_tuner_routes():
+    """Keep sys.modules['routes'] pointing at THIS plugin's routes for these
+    tests, so a runtime `import routes` in a test body resolves correctly
+    regardless of which other plugin's bare-named routes ran first."""
+    prev = sys.modules.get('routes')
+    sys.modules['routes'] = tuner_routes
+    try:
+        yield
+    finally:
+        if prev is not None:
+            sys.modules['routes'] = prev
+        else:
+            sys.modules.pop('routes', None)