feat: implement Tier 3 features — lane queue, credential redaction, token dashboard, xAI, Voyage AI

- Lane Queue: per-session FIFO queue in gateway replacing reject-when-busy (9 tests)
- Credential Redaction: redactConfig() expanded to cover 18+ secret fields (16 tests)
- Web UI Token Dashboard: system.tokenUsage endpoint + Usage page with summary cards
- xAI (Grok) Provider: OpenAI-compatible client with model pricing
- Voyage AI Embeddings: new embedding provider with configurable dimensions (5 tests)
- Update gap analysis: 90→95 match (70%→74%), Tier 3 section marked DONE
- Update state.json: test count 1001→1034, add tier3_completion entry

Total: 1034 tests passing across 85 files, typecheck clean

src/gateway/ui/style.css

@@ -741,6 +741,25 @@ header #status.status-ok {
   margin-top: 24px;
 }
 
+/* ── Usage Page Header ─────────────────────────────────────── */
+
+.usage-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.usage-header .page-title {
+  margin-bottom: 0;
+  padding-bottom: 0;
+  border-bottom: none;
+  flex: 1;
+}
+
+.usage-header .btn {
+  flex-shrink: 0;
+}
+
 /* ── Data Tables ────────────────────────────────────────────── */
 
 table {