refactor(app): carve the plugin loader out of app.js into static/js/ (R3a) (#878)

The first carve, and deliberately the riskiest: app.js IS the plugin loader (the
R0 host rails), so it goes first while the module graph is still one edge deep.

static/js/plugin-loader.js (829 lines) — bodies VERBATIM. app.js 12,217 → 11,439.
Core's first `static/js/` module, exactly as constitution II anticipates.

CLOSURE (measured with acorn, not regex — brace-matching stripped source drifted):
the block at app.js:11246-12031 is contiguous and self-contained. It needs only
TWO things from the rest of app.js, and exports only TWO:
  exports: loadPlugins (the window contract), bootstrapPluginsAndUi (boot)
  inbound: window.showScreen — already the public host contract (constitution II),
           so it is called through `window`, not re-coupled as an import
           _populateVizPicker — injected via configurePluginLoader()

WHY A SEAM, NOT AN IMPORT. plugin-loader must not import app.js: app.js imports
it, so that would close a cycle. I checked whether _populateVizPicker could just
move into the module instead (which would delete the seam entirely) — it drags 9
further symbols (_canRun3D, _autoMatchViz, _showPromotionNag, …), i.e. a whole
viz cluster. That is its own carve, so the seam stays.

THE SEAM'S DEFAULT IS LOUD, ON PURPOSE. A no-op stub is the classic silent
failure for this pattern (see the editor's setHostHooks trap, hit twice): drop the
wiring call and the loader keeps working while the viz picker quietly stops
refreshing — no test, no boot check says a word. The default now console.errors,
so the smoke harness catches it. VERIFIED BY BITE TEST: removing
configurePluginLoader() from app.js surfaces
"[plugin-loader] host seam not configured" at boot. The seam IS exercised on the
plugin-startup path, so an unwired hook cannot pass silently.

no-cycle is now LIVE on core's own graph for the first time. eslint.config.js
gains `static/app.js` + `static/js/**` to the module block — app.js now `import`s,
so parsing it as a script would be a syntax error. VERIFIED BY BITE TEST: making
plugin-loader import app.js back fails with "Dependency cycle detected".

HARNESSES (the R3a note said budget one conversion per carve — it was five):
retargeted capability_inspector_nav, plugin_hydration_wipe,
plugin_loader_script_type, plugin_style_injection, legacy_shim_hits (SPLIT — one
test needs the loader, one still needs app.js) + test_plugin_runtime_idempotence.
legacy_shim_hits was missed by a symbol-name grep because it greps for a code
STRING; only the failing run found it. test_capability_events' NEGATIVE asserts
now span app.js + the loader — carving code out of app.js would otherwise make
them vacuous instead of failing.

VERIFIED: A/B against origin/main in two browsers — mounted plugin screens, 14
loaded plugin scripts, the 3 module plugins injected as <script type="module">,
37 capability participants, 14 shims, window.loadPlugins: IDENTICAL, zero
console/page errors on both. /static/js/plugin-loader.js serves 200; R0 rails
intact (src/main.js 200, conditional GET 304, script_type passthrough).
pytest 2396, node 1032/1032, ESLint 0, Codex 0.

Codex preflight caught a REAL [P1] first pass: static/js/plugin-loader.js was
untracked, so a checkout would have served an app.js importing a nonexistent
module — a failed static import kills the whole module and every window handler
with it. Now tracked.

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

tests/js/legacy_shim_hits.test.js

@@ -71,6 +71,9 @@ test('native audio-mix participant suppresses matching legacy fader and records
 
 const ROOT = path.join(__dirname, '..', '..');
 const APP_JS = path.join(ROOT, 'static', 'app.js');
+// The plugin loader was carved out of app.js into its own module (R3a); the
+// library-provider code below still lives in app.js.
+const PLUGIN_LOADER_JS = path.join(ROOT, 'static', 'js', 'plugin-loader.js');
 const LIBRARY_JS = path.join(ROOT, 'static', 'capabilities', 'library.js');
 
 function source(file) {
@@ -87,7 +90,7 @@ function region(src, needle, length = 1200) {
 }
 
 test('plugin script hydration exposes the current plugin id for legacy registrations', () => {
-    const src = source(APP_JS);
+    const src = source(PLUGIN_LOADER_JS);
     const block = region(src, 'script.src = `/api/plugins/${plugin.id}/screen.js');
     assert.match(block, /window\.feedBack\._loadingPluginId\s*=\s*plugin\.id/);
     assert.match(block, /delete\s+window\.feedBack\._loadingPluginId/);