Add packed join node and positioned-index abstractions

[ezrosent]

Stacked on the DVO policy PR.

This introduces the reusable pieces of the packed generic-join design without switching the executor over:

• arena-packed trie nodes and cursors
• stable positioned-index APIs
• compatibility wrappers that keep the existing executor behavior unchanged

The executor integration remains isolated in #959.

Validation:

• cargo test -p egglog-core-relations
• cargo fmt --check
• cargo clippy -p egglog-core-relations --all-targets -- -D warnings

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.12067% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.14%. Comparing base (f001a63) to head (80431f5).

Files with missing lines	Patch %	Lines
core-relations/src/free_join/packed_trie.rs	97.76%	19 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           codex/dvo-ordering-policy     #969      +/-   ##
=============================================================
+ Coverage                      86.82%   87.14%   +0.31%     
=============================================================
  Files                             95       96       +1     
  Lines                          30659    31667    +1008     
=============================================================
+ Hits                           26620    27596     +976     
- Misses                          4039     4071      +32     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/packed-join-abstractions (80431f5) with codex/dvo-ordering-policy (f001a63)}

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. ↩