feat(voyage): add OPTIONAL_STRING_KEYS path to manifest-yaml — profile_used additive

Step 3 av v4.1-execute (Wave 1, Session 1).

Legg ny eksportert const OPTIONAL_STRING_KEYS = ['profile_used'] parallel
til eksisterende OPTIONAL_KEYS. Utvid parseManifest med ny dispatch-loop
etter OPTIONAL_BOOLEAN_KEYS. Returnerer MANIFEST_OPTIONAL_TYPE hvis
profile_used finnes men ikke er string.

Forskjell fra OPTIONAL_BOOLEAN_KEYS: absence == not-present (NOT defaulted
til false, unlike boolean). Downstream-konsumenter kan dermed skille mellom
unset og empty-string.

Tester (5 nye, baseline 372 → 377):
- OPTIONAL_STRING_KEYS export drift-pin
- profile_used: economy parses successfully (SC #10 forward-compat)
- profile_used: numeric rejected
- absence: field NOT in parsed (string-key semantics)
- profile_used + skip_commit_check + memory_write co-existence

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/voyage/tests/lib/manifest-schema-extensions.test.mjs

@@ -8,7 +8,7 @@
 
 import { test } from 'node:test';
 import { strict as assert } from 'node:assert';
-import { parseManifest, OPTIONAL_KEYS } from '../../lib/parsers/manifest-yaml.mjs';
+import { parseManifest, OPTIONAL_KEYS, OPTIONAL_STRING_KEYS } from '../../lib/parsers/manifest-yaml.mjs';
 
 const BASE = `### Step 1: Cover
 - Manifest:
@@ -90,3 +90,44 @@ test('extension does NOT break REQUIRED_KEYS contract', () => {
     assert.ok(k in r.parsed, `required key ${k} missing after extension`);
   }
 });
+
+// v4.1 Step 3 — OPTIONAL_STRING_KEYS dispatch (profile_used)
+
+test('OPTIONAL_STRING_KEYS exports profile_used', () => {
+  assert.deepEqual(
+    [...OPTIONAL_STRING_KEYS].sort(),
+    ['profile_used'].sort(),
+    'OPTIONAL_STRING_KEYS export drift — pin contract',
+  );
+});
+
+test('profile_used: economy parses successfully (SC #10 forward-compat)', () => {
+  const r = parseManifest(bodyWithExtras('    profile_used: economy'));
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  assert.equal(r.parsed.profile_used, 'economy');
+});
+
+test('profile_used: numeric rejected with MANIFEST_OPTIONAL_TYPE', () => {
+  const r = parseManifest(bodyWithExtras('    profile_used: 42'));
+  assert.equal(r.valid, false);
+  const found = r.errors.find(e => e.code === 'MANIFEST_OPTIONAL_TYPE');
+  assert.ok(found, `expected MANIFEST_OPTIONAL_TYPE, got: ${JSON.stringify(r.errors)}`);
+  assert.match(found.message, /profile_used/);
+  assert.match(found.message, /string/);
+});
+
+test('absence of profile_used: field is NOT in parsed (NOT defaulted, unlike boolean)', () => {
+  const r = parseManifest(bodyOnlyRequired());
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  // Absence semantics differ from boolean: parsed should NOT contain the key
+  assert.equal('profile_used' in r.parsed, false,
+    'profile_used must NOT be auto-defaulted when absent — string-key semantics');
+});
+
+test('profile_used works alongside boolean optional keys (skip_commit_check + memory_write)', () => {
+  const r = parseManifest(bodyWithExtras('    skip_commit_check: true\n    memory_write: true\n    profile_used: balanced'));
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  assert.equal(r.parsed.skip_commit_check, true);
+  assert.equal(r.parsed.memory_write, true);
+  assert.equal(r.parsed.profile_used, 'balanced');
+});