Add join-tail execution state

[ezrosent]

This PR is part of an eight-PR stack that jointly replaces #959.

Stack base: #969

Replacement stack:

1. Add small residual join indexes #993 — small residual indexes
2. Add packed join root caches #994 — packed root caches
3. Add prepared join index state #995 — prepared index state
4. Add packed join probe abstractions #996 — packed probe abstractions
5. Add join-tail execution state #997 — join-tail execution state
6. Use packed arena tries for generic joins #998 — executor integration
7. Compact buffered free-join updates #999 — compact frame updates
8. Lazily prepare packed join state #1000 — lazy preparation

Validation on the complete stack:

• cargo fmt --check
• cargo nextest run -p egglog-core-relations
• 113 tests passed

Layer 5 of 8.

Adds join-tail analysis, binding-state projection, ordering helpers, and worker-local state in a dedicated implementation file. The executor wiring is intentionally deferred to #998.

Non-test diff: +633 / -0.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.37%. Comparing base (5d69f4c) to head (3c94cc2).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           codex/packed-trie-split-04-probes     #997      +/-   ##
=====================================================================
+ Coverage                              87.34%   87.37%   +0.03%     
=====================================================================
  Files                                     96       96              
  Lines                                  32369    32369              
=====================================================================
+ Hits                                   28273    28284      +11     
+ Misses                                  4096     4085      -11     

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[codspeed]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/packed-trie-split-05-join-tail (3c94cc2) with codex/packed-trie-split-04-probes (5d69f4c)}

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. ↩

[yihozhang]

Two thoughts:

• materialization_is_live_in_tail may repeat the same analysis even when the variable ordering is unchanged
• Sibling branches do not share analysis results, even though they may share the same (updated) variable ordering from DVO.

The first one is a low-hanging fruit IMO. We can follow up on them and benchmark the actual cost before deciding whether optimization is worthwhile.

[ezrosent]

+1... I think we should look at sharing some of that in a follow-up PR. Definite potential low-hanging fruit.