Migrate to IDF 5.5.3 candidate

Pin PlatformIO packages and toolchains, rebase protected SDK
overrides, and add WebSocket receive regression coverage. Document
isolated candidate validation, archive provenance, and remaining gates.

docs/agent/architecture.md

@@ -78,7 +78,7 @@ UART RX -> serial-service RX stream -> broker task
 
 The broker drains serial input even with no clients. A full client output stream drops only that client's copy and updates drop counters; it does not block UART reception or other clients.
 
-Binary serial WebSocket output uses the IDF-5.5.0-pinned `web_httpd_ws_send_binary` adapter: one owner-only session-override send of a bounded header+payload copy (516 bytes of local scratch, 512-byte payload). Existing generation validation and one outstanding work item per slot remain required. Non-full sends immediately replace that session's send override with a reject-only guard, mark it closing and shut down the socket before deferred cleanup; this prevents automatic SDK control replies from reentering TLS after incomplete output. HTTPD retains TLS destruction ownership. Text/control and admin output retain the SDK sender. One send call does not imply one TLS record, packet or peer receipt; the user signed off drop-free230400-baud full-client-mix operation at160MHz. Runtime stack-margin validation remains a separate follow-up; evidence and acceptance scope are in `current-state.md`.
+Binary serial WebSocket output uses the IDF-5.5.3-pinned `web_httpd_ws_send_binary` adapter: one owner-only session-override send of a bounded header+payload copy (516 bytes of local scratch, 512-byte payload). Existing generation validation and one outstanding work item per slot remain required. Non-full sends immediately replace that session's send override with a reject-only guard, mark it closing and shut down the socket before deferred cleanup; this prevents automatic SDK control replies from reentering TLS after incomplete output. HTTPD retains TLS destruction ownership. Text/control and admin output retain the SDK sender. One send call does not imply one TLS record, packet or peer receipt; the user signed off drop-free230400-baud full-client-mix operation at160MHz. Runtime stack-margin validation remains a separate follow-up; evidence and acceptance scope are in `current-state.md`.
 
 Active-client counter snapshots expose ID/type/pending/HWM/UART/queued/read/dropped. HWM is maintained independently of web tracing and counter clear seeds current occupancy. Read means transport handoff, not peer receipt. Disconnect removes the client row while global totals retain traffic and unread-output discards. Independent default-disabled web performance capture uses two fixed slot records and nonwrapping epoch/generation fences; toggles fence in-flight samples, disable freezes aggregates, clear preserves enable state. Binary-only timestamps bound reservation-entry to callback-entry before locking, synchronous send calls and completion to broker-read return; idle and intervening work prevent scheduler-only/backlog-at-completion claims. No new instrumentation allocations or scheduling/buffer changes. Exact fields, overhead and capture contracts: [throughput diagnostics](../web_throughput_diagnostics.md).
 
@@ -108,7 +108,7 @@ TinyUSB callbacks enqueue/copy data and state; the transport task owns broker li
 
 Cookie login/logout replaces Basic/cache. Digest-only records carry copied principals, CSRF state, absolute expiry and nonreused originating-session IDs. Strict same-origin/CSRF mutations and session/principal checks gate admission; logout invalidates its session before transport cleanup, account mutations invalidate only the affected account, and ongoing currentness is authoritative. Authentication initialization failure gates HTTPS; failed start/accepted stop wipes records. A non-consuming quota/epoch check rejects exhausted login requests before body receive; verification reservation remains post-parse and only that reservation charges the existing fixed window. RNG/SHA/database calls run outside short spinlocks with post-call epoch/identity revalidation. [Authentication contract](../web_administration.md#authentication-and-admission).
 
-`web_httpd_adapter` is the sole private IDF 5.5.0 boundary for duplicate headers, admission-before-101, consumed-scratch wiping, staged optional URI registration, combined binary sends and owner-only idle sweeps. Re-audit its version guard on SDK upgrades. HTTPD debug logging must not expose headers/tickets. `web_diagnostics` independently observes public post-TLS callbacks using six metadata records and a default-disabled 32-event ring; it cannot see preaccept/in-progress/failed TLS. [Admission diagnostics](../web_admission_diagnostics.md).
+`web_httpd_adapter` is the sole private IDF 5.5.3 boundary for duplicate headers, admission-before-101, consumed-scratch wiping, staged optional URI registration, combined binary sends and owner-only idle sweeps. Re-audit its version guard on SDK upgrades. The 5.5.3 rebase retains private-layout/ownership contracts and adds a pinned WS receive correction so negative fixed-header reads cannot pass unsigned length comparisons; see [rebase evidence](../idf_553_rebase_review.md). HTTPD debug logging must not expose headers/tickets. `web_diagnostics` independently observes public post-TLS callbacks using six metadata records and a default-disabled 32-event ring; it cannot see preaccept/in-progress/failed TLS. [Admission diagnostics](../web_admission_diagnostics.md).
 
 `web_httpd_idle` uses one one-second timer, six rows and at most one queued owner probe. Current-owner shutdown follows 15 seconds of observed ordinary idle, exempting actual WebSockets/async/pending input. Owner delays prevent hard timeout guarantees; accepted-but-lost work stays reserved until successful destruction, failed stop retains ownership. [Idle lifecycle contract](../https_idle_cleanup.md).