fix(voyage): coerce brief_version to string + quote template + update doc pin (closes #8 #11)

v5.1.0 shipped with an unquoted brief_version: 2.1 in trekbrief-template.md.
parseScalar coerced it to Number 2.1, and the sequencing gate guarded on
typeof === 'string', silently bypassing BRIEF_V51_MISSING_SIGNALS.

Three-part atomic fix:
- brief-validator.mjs:87+149 now accepts both string and number forms via
  String(fm.brief_version) coercion.
- trekbrief-template.md quotes the value so new briefs parse as String.
- doc-consistency.test.mjs pins the QUOTED form going forward.

Three regression tests added in brief-validator.test.mjs.

plugins/voyage/tests/validators/brief-validator.test.mjs

@@ -218,3 +218,35 @@ test('validateBrief — phase_signals with unknown phase rejected', () => {
   assert.equal(r.valid, false);
   assert.ok(r.errors.find(e => e.code === 'BRIEF_INVALID_PHASE_SIGNAL_PHASE'));
 });
+
+// --- v5.1.1 regression: YAML-number bypass closed ---
+// Findings 3c834097 + df1435a2: v5.1.0 shipped with an unquoted `brief_version: 2.1`
+// template. parseScalar coerces unquoted "2.1" to Number 2.1, and the original gate
+// guarded `typeof === 'string'`, silently bypassing the sequencing check. v5.1.1
+// coerces via String() so both shapes trigger the gate.
+
+test('validateBrief — v5.1.1: UNQUOTED brief_version 2.1 without signals triggers gate', () => {
+  const t = GOOD_BRIEF.replace('brief_version: "2.0"', 'brief_version: 2.1');
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, false);
+  assert.ok(
+    r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'),
+    `gate must fire for unquoted brief_version: 2.1 (YAML Number); errors=${JSON.stringify(r.errors)}`,
+  );
+});
+
+test('validateBrief — v5.1.1: QUOTED brief_version "2.1" without signals triggers gate (regression guard)', () => {
+  const t = GOOD_BRIEF.replace('brief_version: "2.0"', 'brief_version: "2.1"');
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, false);
+  assert.ok(r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'));
+});
+
+test('validateBrief — v5.1.1: UNQUOTED brief_version 2.1 WITH phase_signals is valid (positive case)', () => {
+  const t = GOOD_BRIEF
+    .replace('brief_version: "2.0"', 'brief_version: 2.1')
+    .replace('source: interview\n', `source: interview\n${SIGNALS_BLOCK}`);
+  const r = validateBriefContent(t, { strict: true });
+  assert.equal(r.valid, true, JSON.stringify(r.errors));
+  assert.ok(!r.errors.find(e => e.code === 'BRIEF_V51_MISSING_SIGNALS'));
+});