refactor(app): the host seam + carve section practice out of app.js (R3a) (#887)

static/js/host.js (99) + static/js/section-practice.js (1,214).
app.js 9,461 → 8,409.

THE FIRST SLICE OUT OF THE STRONGLY-CONNECTED CORE. What is left in app.js is not
a tree, it is a cycle: seeding a dependency closure from section-practice, from
loops, from count-in, or from the JUCE seek shim all return the SAME 178-function
set, and setLoop() and practiceSection() call each other directly. No closure-based
carve can cut it at any seed. So it is cut BY NAME, and the calls back into app.js
go through a host seam.

  61 functions + its own 24 _sectionPractice*/_sectionParents* scalars (read nowhere
  else) move out. 11 hooks come back in. 4 of those are read-only GETTERS —
  loopA/loopB/_audioSeekGen/_loopMutationGen are only ever READ here, never written,
  so app.js keeps owning them and NO state container is needed (a 977-site lift
  avoided).

app.js used to reach IN and reset the module's state by hand (clearLoop() zeroed the
selection; changeArrangement() invalidated the parent count). It cannot now — an
imported binding is read-only — so those are exported as resetSelection() and
invalidateParentCount(). Strictly better: the module owns its own invariants instead
of trusting two callers on the far side of the file to zero the right three fields.

═══ THE SILENT-NO-OP PROBLEM, SOLVED ═══
The obvious host seam is an object of no-op defaults. That is a TRAP and we walked
into it once: the plugin loader's seam defaulted populateVizPicker to `() => {}`, so
a dropped wiring line would have left the viz picker quietly not refreshing with NO
test, boot check, or bot noticing. Two layers stop it here:

  1. RUNTIME — host.js is a Proxy with NO defaults and NO stubs. Reading an unwired
     hook THROWS. An unwired hook cannot degrade into a no-op because there is
     nothing to degrade INTO. configureHost() also rejects a non-function at WIRE
     time, and refuses to run twice.

  2. STATIC — tests/js/host_contract.test.js asserts the hooks the modules USE are
     exactly the hooks app.js WIRES. This is the layer that matters: a runtime throw
     only fires if the broken path executes, and the whole danger of a seam is the
     paths that never run in a smoke test. VERIFIED TO BITE in all three drift
     directions: drop a hook from configureHost -> fails; rename host.setLoop in the
     module -> fails; wire a hook nobody uses -> fails.

Writing that guard took three tries and each failure is instructive: (a) the
configureHost regex anchored `});` at column 0, ran past the indented close, and
swallowed app.js's 66-name window contract — 77 "hooks"; (b) an import-stripping
regex with `[\s\S]*?` ate 14,000 characters INCLUDING the drift the bite test was
meant to catch — a guard with a hole is worse than no guard, because you trust it;
(c) `host.js'` in the import path backtracked from `js` to a "hook" called `j`.
The bite tests are what surfaced all three.

CODEX FOUND A REAL RACE [P2]. configureHost() was inside the async boot function,
after several awaits — but the window handlers (onPhraseNext, …) go live during
app.js's SYNCHRONOUS module evaluation. A user clicking one in that window would hit
"[host] … was read before configureHost() ran". It is now a bare top-level statement
sitting immediately before the window contract, so the seam is always wired before a
handler can be reached. Verified live: invoking a handler 1.2s in — well before the
boot awaits settle — works.

VERIFIED. A/B against origin/main in two browsers with a REAL song loaded: popover
toggle, practice-mode change, phrase-next, and clearLoop (all of which cross the seam
— setLoop/clearLoop/_audioTime/loopA/loopB) — IDENTICAL, zero page errors. Since an
unwired hook throws, a live app is itself proof the seam is wired.

Harnesses: section_practice_dismiss retargeted; loop_api's clearLoop sandbox gains a
resetSelection SPY (not a stub) and ASSERTS it fires — the guarantee is still tested,
just through the seam.

pytest 2396, node 1040/1040, ESLint 0 (no-cycle clean), tailwind clean, Codex 0.

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

tests/js/loop_api.test.js

@@ -44,10 +44,18 @@ function buildSandbox() {
     const seekCalls = [];
     const sectionPracticeModeCalls = [];
     const transportEvents = [];
+    // clearLoop() used to zero section-practice's three selection scalars by hand.
+    // They now live in static/js/section-practice.js, which owns them, so clearLoop
+    // calls its exported resetSelection() instead. This is a SPY, not a stub — the
+    // test below still asserts the reset happens, it just asserts it through the
+    // seam rather than by reaching into someone else's state.
+    const resetSelectionCalls = [];
     const sandbox = {
         seekCalls,
         sectionPracticeModeCalls,
         transportEvents,
+        resetSelectionCalls,
+        resetSelection: () => resetSelectionCalls.push(true),
         // Mutable state (declared as `var` in eval prelude so it lives on
         // the sandbox global and the extracted functions can read/write).
         // The actual values are set below.
@@ -201,7 +209,7 @@ test('setLoop rejects b <= a', async () => {
     await assert.rejects(() => sandbox.__setLoop(10, 5), /b > a/);
 });
 
-test('clearLoop resets loopA/loopB to null', async () => {
+test('clearLoop resets loopA/loopB to null (and asks section-practice to drop its selection)', async () => {
     const src = fs.readFileSync(APP_JS, 'utf8');
     const sandbox = buildSandbox();
     loadFunctions(sandbox, src);
@@ -211,6 +219,11 @@ test('clearLoop resets loopA/loopB to null', async () => {
     const { loopA, loopB } = sandbox.__getLoop();
     assert.equal(loopA, null);
     assert.equal(loopB, null);
+    assert.equal(
+        sandbox.resetSelectionCalls.length, 1,
+        'clearLoop must ask section-practice to drop its selection (it used to zero the '
+        + 'scalars by hand; the module owns them now)',
+    );
     assert.equal(sandbox.sectionPracticeModeCalls.length, 1);
     assert.equal(sandbox.sectionPracticeModeCalls[0].on, false);
     // Field-wise: vm-context objects break deepStrictEqual across realms.