[igorls]

Adds the protocol and flow-control foundations for #294 after the shared history extraction in #295. This slice provides codec and credit modules; the native endpoint and session integration remain subsequent work.

• Validate bounded JSON envelopes, canonical decimal-string identifiers, exact sequence metadata and receipt structures, nondecreasing blocks within a frame, UTF-8 byte limits, duplicate/escaped keys, Unicode and finite numbers. Arbitrary contract data remains opaque and wider/compound delta identities retain their strings.
• Pin client 346c473f55979ff2dd841aa2bbba4bcfebc49ef7. Execute all 37 shared wire vectors and verify the frozen protocol, manifest, 24 state scenarios and two network-session fixtures by SHA-256. Session/network fixture execution awaits the server integration.
• Account for per-connection/per-subscription counts and complete encoded bytes. Fence serialization reentry, reserve before synchronous transport write, commit only on explicit acceptance, release failed writes, reject stale/duplicate/out-of-order ACK credit and bound active/lifetime identities. Only the next committed frame settles its retained costs.
• Document remaining bounded queues/query memory, page splitting, deadlines, independently deliverable controls, real replay anchors, confirmed cleanup and fork/relay handling.

Validation at e8dd91a7693ad22fbfd4d98c0c2ec46461f6256a: Bun 1.4.4 whole-project check (256 files), TypeScript 7.0.2 artifact build and all 91 focused history/filter/codec/credit tests pass on Node 22.23.3 and 24.19.0. Regressions cover serialization reentry/error/retirement, exact credit release, malformed known metadata, opaque payloads and unrelated root counts, and descending frame blocks. CI includes the native suites; independent final Standards/Spec reviews found no remaining scoped issues; artifacts were independently hash-verified. All nine final-head CI jobs passed. Merged as d665dc6a6211079cacd2bee6bba0a44c5f4ba321; its tree equals the reviewed head. Post-merge CI is running.

This PR registers no route or operator default. #294 and #231 remain open for endpoint, retained-history, proxy, recovery and real server/client acceptance. No throughput, peak-memory recovery or Bun indexing-runtime qualification is claimed.

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