Move Map containers to sequence storage

[ezrosent]

Summary

• Encode maps as [rebuild mask, sorted key/value pairs...].
• Implement lookup and editing directly over the canonical sequence using binary search and slice splicing.
• Preserve selective key/value rebuilding and last-old-key-wins behavior when rebuilt keys collide.
• Cover codec round trips, rebuild masks, collisions, predictions, and last-write-wins updates.

Stack

Draft, stacked on #979. This is PR 7 of 12 in the sequence-backed container stack.

Validation

Validated on the complete 12-commit stack:

• cargo nextest run --workspace: 1,362 tests passed
• cargo test --workspace --doc
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.88991% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.84%. Comparing base (d8a18cf) to head (4a80623).

Files with missing lines	Patch %	Lines
src/sort/map.rs	92.88%	31 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           codex/inline-containers-06-vec     #980      +/-   ##
==================================================================
+ Coverage                           87.69%   87.84%   +0.15%     
==================================================================
  Files                                  98       98              
  Lines                               37945    38368     +423     
==================================================================
+ Hits                                33274    33706     +432     
+ Misses                               4671     4662       -9     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/inline-containers-07-map (4a80623) with codex/inline-containers-06-vec (d8a18cf)}

Footnotes

1. 227 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩