fix(#838): bound broker reply deadlines + fail-close empty-read; de-flake acceptance harness (#839)

Bound broker reply deadlines (separate read/lock/send budgets, BROKER_BUSY-before-mutation, fresh post-handle send budget → closes drop-after-commit window); fail-close empty/truncated read → GATE_UNAVAILABLE deny. De-flakes the 1917 acceptance surface. terra CODE APPROVE (pi) + Opus SECREV APPROVED (claude) — RoR comment 18143. Promote-lease-lost-ACK residual = fail-safe two-generals observability-gap, routed to WI-3 D2-v5 as named-disclosed-bounded-residual (route i). Gate-16 3-principal author=gpt-sol.

closes #838

packages/mosaic/vitest.config.ts

@@ -7,9 +7,10 @@ export default defineConfig({
     testTimeout: 30_000,
     coverage: {
       provider: 'v8',
-      include: ['src/commands/skill.ts'],
+      include: ['src/commands/skill.ts', 'src/lease-broker/broker-test-client.ts'],
       reporter: ['text', 'json-summary'],
       thresholds: {
+        perFile: true,
         statements: 85,
         branches: 85,
         functions: 85,