Move MultiSet containers to sequence storage

[ezrosent]

Summary

• Add a compact canonical encoding using sorted (value, count) entries rather than repeated values.
• Implement reads, edits, and multiset algebra directly over counted sequence slices.
• Preserve multiplicities across rebuild collisions and enforce language-level cardinality limits.
• Cover count boundaries, large multiplicities, overflow rejection, algebra, predictions, and nested rebuilds.

Stack

Draft, stacked on #981. This is PR 9 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 92.46575% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.77%. Comparing base (0481cf8) to head (08d400c).

Files with missing lines	Patch %	Lines
src/sort/multiset.rs	92.46%	55 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           codex/inline-containers-08-set     #982      +/-   ##
==================================================================
- Coverage                           87.87%   87.77%   -0.10%     
==================================================================
  Files                                  98       98              
  Lines                               38738    39312     +574     
==================================================================
+ Hits                                34040    34508     +468     
- Misses                               4698     4804     +106     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/inline-containers-09-multiset (08d400c) with codex/inline-containers-08-set (0481cf8)}

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. ↩