refactor(sctp): identify outbound messages independently of wire sequences

[realzoberg]

Outbound message abandonment currently locates retained fragments through TSN ranges and identifies pending tails through queue positions. Give each outbound message one association-local MessageId, so its identity survives fragment removal and cannot refer to a later message that reuses the same wire TSN.

• Assign the same local identity to every fragment at enqueue time. DATA encoding and public APIs stay unchanged.
• Retire retained fragments and the pending tail by identity, preserving checked, atomic pending-tail validation and exactly-once buffer release.
• Index only fragmented, partially reliable messages. Whole-message candidates use their TSN plus an identity check; reliable messages need no index entry.
• Preserve fix(sctp): defer Rexmit abandonment until retransmission #242's candidate-only retry checks, T3 ordering, timer behavior and error handling.

The memory tradeoff is an 8-byte optional identity field per DATA chunk and an additional TSN index for fragmented, partially reliable messages. Index entries leave with cumulative acknowledgments; a single retained TSN is stored inline.

[codecov]

Codecov Report

❌ Patch coverage is 96.82540% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.80%. Comparing base (784e464) to head (19e50c0).

Files with missing lines	Patch %	Lines
rtc-sctp/src/association/message_selection_test.rs	97.05%	2 Missing ⚠️
rtc-sctp/src/queue/payload_queue.rs	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
- Coverage   87.85%   87.80%   -0.05%     
==========================================
  Files         499      501       +2     
  Lines       72540    72646     +106     
==========================================
+ Hits        63728    63790      +62     
- Misses       8812     8856      +44     

Flag	Coverage Δ
aws-lc-rs	87.64% <96.82%> (-0.05%)	⬇️
ring	87.80% <96.82%> (+0.01%)	⬆️

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