chore: resolve actionable TODO/FIXME comments

[nightness]

Summary

Cleans up four concrete TODO/FIXME items found in a codebase-wide audit:

File	Change
rtc-turn/src/proto/evenport.rs:37	FIXME? (1 << 8) - 1 -- the existing 0b10000000 value is correct per RFC 5766 §14.6 (only the MSB is the R flag); replaced the FIXME with an explanatory comment
rtc/src/rtp_transceiver/rtp_sender/rtp_codec.rs:145	Replace TODO: unicode case-folding + .to_uppercase() with eq_ignore_ascii_case() -- allocation-free and correct for ASCII-only MIME types
rtc-sctp/src/endpoint/mod.rs:133	Replace DoS TODO with a comment documenting the existing concurrent_associations protection and referencing RFC 4960 §5.1.3 for a future stateless cookie implementation
rtc/src/peer_connection/configuration/setting_engine.rs:175	Replace misleading Re-enable it to QueryOnly TODO with a comment explaining the opt-in design (both the core and the async wrapper require explicit configuration)

Test plan

• cargo build passes
• cargo test -p rtc passes (246 tests)
• cargo test -p rtc-sctp passes
• cargo test -p rtc-turn passes
• cargo fmt --check passes
• cargo clippy passes

Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Resolves four actionable TODO/FIXME items discovered in an audit by replacing them with clarified documentation and a small correctness/performance improvement in MIME type matching.

Changes:

• Replace MIME type .to_uppercase() comparisons with eq_ignore_ascii_case() for allocation-free ASCII-only comparisons.
• Convert TODO/FIXME markers into explanatory comments with relevant RFC references.
• Clarify mDNS default behavior as opt-in with additional wrapper configuration requirements.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
rtc/src/rtp_transceiver/rtp_sender/rtp_codec.rs	Uses eq_ignore_ascii_case() for MIME type matching and removes a misleading TODO.
rtc/src/peer_connection/configuration/setting_engine.rs	Replaces a misleading TODO with an explanation of the opt-in mDNS design.
rtc-turn/src/proto/evenport.rs	Replaces an uncertain FIXME with an RFC-backed explanation of the R-flag bit mask.
rtc-sctp/src/endpoint/mod.rs	Replaces a DoS TODO with documentation of existing protections and RFC references for future hardening.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.16%. Comparing base (9feb4a3) to head (501d281).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   71.17%   71.16%   -0.01%     
==========================================
  Files         442      442              
  Lines       67330    67330              
==========================================
- Hits        47922    47916       -6     
- Misses      19408    19414       +6     

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

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[nightness]

Rebased onto upstream/master so this PR contains only its own changes. Previous branch structure caused merge conflicts when PRs were merged in sequence. Each PR is now independently mergeable.