feat(extension): bound native messaging framing

[seonghobae]

Partial implementation of #27, stacked on live PR #82 (feat/native-messaging-host-authority).

Buyer/security gap

PR #82 establishes exact extension-to-native-host authority but intentionally leaves native-messaging stdio framing and process integration unimplemented. A hostile or buggy host must not be able to make OriginWeave accept an unbounded, truncated, overlong, directionally oversized, or invalid-text native-messaging frame before a later adapter parses JSON or treats payload bytes as untrusted observation data.

Current dependency / stack state

The independently resolved live prerequisite is exact #82 head 427d2f32431139dc7ed59e60df00fd9d0c4eeba0. Current exact #154 head is d2e1ae8d654703b76897db202980fec82d26babc.

The prerequisite moved after this Draft was opened by adding public native-host/request identity getters and their tests. The previous #154 head 5a66d3687092aedcd83bb04ae06b0051087e2892 had not incorporated those production getters while its inherited/native-messaging tests expected them. Exact CI run 31906244904, Rust contracts job 95064418409, therefore failed first at the locked workspace check with four E0599 missing-method errors for extension_id / host_name.

The current head non-destructively merges the live prerequisite as a second parent while preserving this PR's framing delta. Fresh ancestry comparison reports #154 ahead of #82 with behind_by=0 and merge base exactly 427d2f32431139dc7ed59e60df00fd9d0c4eeba0. No prerequisite check, review, status, mergeability result, or synthetic merge result is transferred.

Keep Draft while #82 remains active.

RED -> RCA -> GREEN

1. Test-first head cdf290eed42e3d2cf1d3c8f9532f7f97452c6509 established the missing framing contract. CI run 31797616110 reached cargo check --locked --workspace --all-targets and failed because the framing API did not yet exist.
2. Production added only the bounded framing primitive: direction-specific 1 MiB host-to-browser and 64 MiB browser-to-host payload ceilings, a native-endian 32-bit payload-length prefix, rejection of oversized payloads before encoder allocation, rejection of oversized advertised lengths before decoder slicing, exact total-frame-length equality, UTF-8 validation for text frames, and standard source-free error integration.
3. Implementation head b5ec0ae0abe00b3d051f0a868e57f883eb13829c exposed canonical rustfmt differences; 9e0bdb05bfbb023f9739e41f5c877776eae7cb14 applied them and was exact-green against its then-current base.
4. Later feat(extension): bound native messaging host authority #82 movement exposed the stack-drift compile failure described above. Current head d2e1ae8d654703b76897db202980fec82d26babc repairs the dependency topology without force-push, destructive rebase, or semantic broadening.

Current exact-head evidence

On unchanged exact head d2e1ae8d654703b76897db202980fec82d26babc against exact live prerequisite 427d2f32431139dc7ed59e60df00fd9d0c4eeba0:

• CI run 31908472261: success;
• Rust contracts job 95069845044: repository contracts, canonical formatting, locked workspace/all-target check, full tests, strict Clippy, and rustdoc success;
• Production coverage job 95069845104: exact owned-production function/line/region/branch measurement and enforcement success;
• Manifest V3 Compatibility run 31908472054: success;
• formal reviews: none currently returned;
• inline review threads: none currently returned; and
• GitHub reports the PR open, mergeable, and Draft.

Security/SAST workflows not generated for this stacked Draft base are absent and are not represented as passing evidence. Skipped diagnostic-upload steps are likewise not promoted as passing gates. No predecessor-head, prerequisite-head, queued, pending, skipped, cancelled, absent, stale, status-only, model-only, or synthetic evidence is promoted as current proof.

Truth boundary

This slice is framing/resource-boundary work only. It does not spawn or sandbox a native host, validate an installed host manifest or operating-system registration owner, parse or trust JSON, authenticate message provenance, authorize Agent actions, expose secrets, persist authority, or claim complete Chromium native-messaging integration. A later trusted process adapter must compose exact host authority, registered-host/process ownership, bounded binary stdio, JSON parsing, and untrusted-message classification without turning Chrome nativeMessaging permission into Agent authority.

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