docs/INVARIANTS.md: add outbound no-replay invariant

[kcsongor]

Summary by CodeRabbit

• Documentation
  • Added INV-032: per-transceiver outbound emission uniqueness to prevent duplicate outbound message emissions.
  • Described enforcement approaches across platforms, including per-transceiver release tracking and queue removal to stop re-emission.
  • Defined new error codes for detecting and reporting duplicate emission attempts.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds INV-032 to docs/INVARIANTS.md: an invariant requiring that each transceiver may emit a specific outbound message only once, with Solana/Sui using a released bitmap and EVM using atomic queue deletion; introduces related error codes and references to updated outbox/queue logic.

Changes

Cohort / File(s)	Summary
Outbound Message Invariant Documentation docs/INVARIANTS.md	Adds INV-032: per-transceiver outbound emission uniqueness. Documents enforcement mechanisms (released bitmap for Solana/Sui, atomic queue deletion for EVM), mentions new error codes (MessageAlreadySent, EMessageAlreadySent), and references OutboxItem.released, try_release(), and completeOutboundQueuedTransfer().

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 I nibble at the log with care,
One hop, one send — no duplicate dare.
Bitmaps and queues keep mischief at bay,
INV-032 guards the one-way.
Hooray for messages sent just once! 🎉

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely describes the main change: adding an outbound replay protection invariant to the INVARIANTS.md documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch cs/ntt-outbound-replay-invariant

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@docs/INVARIANTS.md`:
- Around line 280-289: Update INV-032 to clarify that the same
OutboxItem.released bitmap implementation underpins INV-004, INV-005 and INV-032
(hence reuse of the MessageAlreadySent / EMessageAlreadySent error names) and
explicitly document that EVM enforces the invariant differently:
completeOutboundQueuedTransfer() relies on atomic queue deletion and absence of
an entry (results in OutboundQueuedTransferNotFound on a second attempt) rather
than emitting a MessageAlreadySent error; reference OutboxItem.released,
try_release(), completeOutboundQueuedTransfer(), MessageAlreadySent,
EMessageAlreadySent, and OutboundQueuedTransferNotFound in the note so readers
understand shared implementation vs distinct EVM enforcement.