fix: bound Cursor Connect frame buffering

[Ingwannu]

Summary

• keep uint32-compatible outbound Connect frames while limiting inbound payloads to 32 MiB
• reject an oversized declared length as soon as the five-byte header arrives
• incrementally complete one pending frame instead of repeatedly concatenating it with unrelated following frames
• fail a turn when EOF leaves an incomplete Connect frame

Part of #820. The 32 MiB inbound limit matches the repository existing framing-hardening design note (devlog/_fin/350_cursor-provider-add/116_phase33-connect-framing-robustness-plan.md).

Verification

• bun test tests/cursor-framing.test.ts tests/cursor-hardening.test.ts tests/cursor-live-transport.test.ts (45 pass)
• bun run typecheck
• bun run privacy:scan

Compatibility

Normal split headers, split payloads, and multiple frames per chunk preserve their order. A legitimate inbound frame larger than 32 MiB now fails explicitly; outbound framing remains unchanged.

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[Ingwannu]

CI follow-up: the initial macOS failure was the same Bun 1.3.14 Trace/BPT trap / bun.report signature seen on unrelated PR #841. The failed job passed on rerun, while the other platform jobs were already green. No code change was needed.

[lidge-jun]

Maintainer triage: this is a narrow and valuable hardening change. It bounds inbound Cursor Connect frame buffering and rejects incomplete streams instead of allowing unbounded or ambiguous framing state.

The branch currently conflicts with dev, so it needs a rebase before it can land. It also shares the Cursor transport/docs surface with #845; please land the two PRs in a defined order (or consolidate them) to avoid overlapping changes and documentation drift.

Because this is co-maintainer-authored, an independent non-author review is still required before landing. Thanks for keeping the scope focused.