Add prepared join index 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 3 of 8.

Adds execution-scoped prepared-index state, continuation caches, and tail-mask preparation in a dedicated implementation file. The executor wiring is intentionally deferred to #998.

Non-test diff: +473 / -0.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.36%. Comparing base (b949610) to head (8046fd0).

Additional details and impacted files

@@                           Coverage Diff                            @@
##           codex/packed-trie-split-02-packed-cache     #995   +/-   ##
========================================================================
  Coverage                                    87.36%   87.36%           
========================================================================
  Files                                           96       96           
  Lines                                        32389    32389           
========================================================================
+ Hits                                         28296    28297    +1     
+ Misses                                        4093     4092    -1     

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[codspeed]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/packed-trie-split-03-prepared-index (8046fd0) with codex/packed-trie-split-02-packed-cache (b949610)}

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]

I realize I should have read this function first, which explains the high-level structure of this file. Maybe leave a note in the top-level doc that someone new to this file should start with this function

[yihozhang]

I find this struct hard to understand, probably because all of its uses are in execute.rs (in a later PR) via direct field access, and it lacks any associated methods to encapsulate operations over PreparedIndexSlot.