fix(p2p): harden direct-P2P framing and ingress admission

[trvon]

Summary

Stacked draft child of #74. Closes four direct-P2P wire/ingress findings from the deep review of the #68 stack, without merging or deploying anything.

1. Pre-authentication JSON recursion DoS (P1) — parseJsonFrame now rejects control frames nested deeper than 32 levels before nlohmann recursively descends them, so a ~32k-deep [[[…]]] hello cannot overflow the session-thread stack before any trust decision.
2. Control-frame cap vs 256-writer state frame (P1) — kP2pMaxControlFrameBytes raised 64 KiB → 512 KiB to cover the worst-case authenticated state frame (~300 KiB: version vector + seen watermark + 256 commitments + quarantined set).
3. Direct-P2P ingress bypasses the resource governor (P2) — MemorySyncLoop::applyDeltas now runs beforeDirectIngress() (unconditional canAdmitRemoteWork + isCancelled), so the daemon's ResourceGovernor can throttle a peer flooding deltas on the new replication path even though the backend is local.
4. Wire-byte under-counting (P2) — delta/bootstrap accounting now uses the exact received frame size (readJsonFrame().payloadBytes) instead of a compact re-serialization, so whitespace/duplicate-key padding from a peer counts against maxWireBytesPerSession/maxSnapshotWireBytes.

Confirmed non-issue (no change): the cold-bootstrap snapshot path is correctly multi-writer — each winner's origin is authenticated by its own schema-v4 writer signature (validateBootstrapWinner → validateDirectDelta → writerAuth_->verify) plus the witness root digest, so it does not require the single-writer origin == peerNodeId check used on the delta-continuation path.

AI disclosure

Extensive AI assistance was used for implementation, test generation, and adversarial review. All changes were locally inspected and validated; the umbrella #68 remains blocked by the decision-grade live-performance gate.

Stack

• Base: fix/p2p-operator-validation / test(p2p): add direct control fuzz harnesses #74 at 87e298e04415fbfb2ed5f06cd40712c428186f94
• Head: fix/p2p-wire-hardening at 24923e95835bf31633ff9c54b46fe2a5c0b1e55c
• Umbrella: feat(p2p): add authenticated direct replication #68

Do not merge or deploy this child independently.

[Copilot]

🔵 Needs a closer look

It changes security-critical pre-auth P2P parsing/framing and ingress admission behavior, which warrants a final human review.

Pull request overview

This PR hardens the direct-P2P replication path by adding pre-auth JSON depth rejection, correcting receiver-side wire-byte accounting to use exact frame sizes, enforcing resource-governor admission for direct delta ingress, and raising the control-frame cap to accommodate worst-case authenticated state frames.

Changes:

• Add a JSON nesting-depth pre-scan in control-frame parsing and introduce readJsonFrame() to retain exact received payload size for accounting.
• Update delta/bootstrap receive paths to account bytes using the original wire size rather than compact re-serialization.
• Gate MemorySyncLoop::applyDeltas() behind a direct-ingress admission check and expand unit coverage for the new behavior; raise kP2pMaxControlFrameBytes and add a transport regression test.

File summaries

File	Description
tests/unit/memory_sync/memory_sync_catch2_test.cpp	Adds a unit test ensuring applyDeltas() rejects work when direct-ingress admission is denied.
tests/unit/daemon/p2p_transport_catch2_test.cpp	Adds a regression test validating large control frames can round-trip after the cap increase.
tests/unit/daemon/p2p_protocol_catch2_test.cpp	Adds a test ensuring handshake control frames reject excessive JSON nesting depth pre-auth.
tests/unit/daemon/p2p_delta_catch2_test.cpp	Adds a test verifying receiver byte accounting includes JSON padding/whitespace on the wire.
src/daemon/p2p/p2p_json.h	Implements JSON nesting depth pre-scan and readJsonFrame() (JSON + exact wire payload bytes).
src/daemon/p2p/p2p_delta.cpp	Switches delta/bootstrap receive accounting to use readJsonFrame().payloadBytes.
include/yams/memory_sync/memory_sync.h	Adds an unconditional direct-ingress admission gate for peer-delivered deltas.
include/yams/daemon/p2p/p2p_transport.h	Raises the control-frame cap to 512 KiB and documents sizing rationale.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite

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