fix(#838): validate complete broker reply frames

docs/scratchpads/838-broker-acceptance-flake.md

@@ -36,3 +36,7 @@ Eliminate high-contention uncaught JSON parse failures in lease-broker and mutat
 - Residual/tripwire: an unavoidable peer disconnect or send failure can still lose acknowledgement after a valid transaction commits. The affected adapter call fails closed. A valid `promote_lease` may nevertheless remain VERIFIED after its acknowledgement is lost; that is authority-observability divergence requiring WI-3/Opus security review rather than expansion of #838. #838 does not add retries or attempt a protocol redesign.
 - Final package evidence: recursive Mosaic dependency build passed; 73 Vitest files / 1,392 tests passed; deadline unit tests 2/2, real runtime tool tests 15/15, launch guard tests 12/12, inventory 14/14, and shell regressions passed.
 - Final coverage: `broker-test-client.ts` 99.24% statements/lines, 86.11% branches, 100% functions under per-file >=85% thresholds. Repository typecheck 42/42, lint 23/23, and format check passed.
+- Independent Codex exact-head review requested one framing fix: a valid frame followed by trailing bytes in a later socket chunk could resolve before the garbage arrived. Security review also flagged complete token-bearing reply bodies in diagnostic properties/logs.
+- Review RED evidence: delayed cross-chunk garbage resolved `{ ok: true }` instead of rejecting, and a truncated `promotion_token` remained in the typed error. The tests use separate timed writes to prevent kernel/event-loop coalescing.
+- Review remediation: the shared client now accumulates through EOF, requires exactly one terminal newline, then parses inside a rejecting error boundary. Diagnostic bodies are capped at 256 bytes, sensitive broker fields are fully redacted, and a SHA-256 digest preserves correlation without credential disclosure.
+- Post-review coverage: `broker-test-client.ts` 99.35% statements/lines, 86.66% branches, 100% functions. Final full suite is 73 files / 1,394 tests plus all Python/shell gates; recursive build, typecheck, lint, and format are green.