test(contracts): Phase 0.a contract snapshots for audio surface

Snapshot the three public surfaces the audio-engine decomposition must not
change (docs/audio-engine-tlc.md Part IV §4): addon export table, audio-bridge
IPC channels, preload audio/audioEffects API keys. contract-check.test.js
diffs regenerated surfaces against the committed snapshots.

result-shapes.json (golden result key/type shapes) is deferred until the
engine_units harness can run the addon against a null device.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/contract-check.test.js

@@ -0,0 +1,34 @@
+// Phase 0.a gate (docs/audio-engine-tlc.md §4): the public audio surface —
+// addon exports, IPC channels, preload API keys — must not change during the
+// decomposition phases. Removals/renames fail here; deliberate additions
+// require regenerating the snapshots in the same commit:
+//   node tests/contracts/extract.js
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert');
+const fs = require('fs');
+const path = require('path');
+
+const { extractAddonExports, extractIpcChannels, extractPreloadApi } = require('./contracts/extract.js');
+
+function loadSnapshot(name) {
+    return JSON.parse(fs.readFileSync(path.join(__dirname, 'contracts', name), 'utf8'));
+}
+
+test('addon export table matches snapshot', (t) => {
+    const current = extractAddonExports();
+    if (!current) {
+        t.skip('slopsmith_audio.node not built');
+        return;
+    }
+    assert.deepStrictEqual(current, loadSnapshot('addon-exports.json'));
+});
+
+test('audio-bridge IPC channels match snapshot', () => {
+    assert.deepStrictEqual(extractIpcChannels(), loadSnapshot('ipc-channels.json'));
+});
+
+test('preload audio/audioEffects API keys match snapshot', () => {
+    assert.deepStrictEqual(extractPreloadApi(), loadSnapshot('preload-audio-api.json'));
+});