Remove legacy container storage

[ezrosent]

Summary

• Delete the DashMap-based container environment, reverse bookkeeping maps, backend dispatch, and legacy-only rebuild machinery.
• Fold the sequence codec contract into the sole ContainerValue trait and make every container an ordinary maintenance participant.
• Use the dependency graph as the authoritative participant-ID allocator.
• Simplify registry, bridge, execution, and testing APIs around the unified representation.

Stack

Draft, stacked on #984. This is PR 12 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 95.23810% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.01%. Comparing base (3be29f9) to head (6ab0cd4).

Files with missing lines	Patch %	Lines
core-relations/src/containers/mod.rs	91.89%	3 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                             @@
##           codex/inline-containers-11-unstable-fn     #985      +/-   ##
==========================================================================
+ Coverage                                   87.39%   88.01%   +0.61%     
==========================================================================
  Files                                          98       98              
  Lines                                       39884    39244     -640     
==========================================================================
- Hits                                        34856    34539     -317     
+ Misses                                       5028     4705     -323     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/inline-containers (6ab0cd4) with codex/inline-containers-11-unstable-fn (3be29f9)}

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. ↩