Move UnstableFn containers to sequence storage

[ezrosent]

Summary

• Encode UnstableFn values as self-describing sequences containing the opaque descriptor, argument count, rebuild masks, and partial arguments.
• Rebuild argument lanes without resolving the descriptor through BaseValues.
• Add prepared functions that resolve and copy partial arguments once for repeated higher-order callbacks.
• Cover multiword masks, malformed encodings, descriptor-free rebuilds, nested dirty propagation, and application after rebuild.

Stack

Draft, stacked on #983. This is PR 11 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 96.38298% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.63%. Comparing base (729c201) to head (83b5f30).

Files with missing lines	Patch %	Lines
src/sort/fn.rs	97.69%	8 Missing ⚠️
src/exec_state.rs	82.14%	5 Missing ⚠️
core-relations/src/containers/sequence.rs	84.00%	4 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##           codex/inline-containers-10-pair     #984      +/-   ##
===================================================================
- Coverage                            87.88%   87.63%   -0.25%     
===================================================================
  Files                                  103      103              
  Lines                                39568    39936     +368     
===================================================================
+ Hits                                 34773    34997     +224     
- Misses                                4795     4939     +144     

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[codspeed]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/inline-containers-11-unstable-fn (83b5f30) with codex/inline-containers-10-pair (729c201)}

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. ↩