add heuristic reordering

[StarGazerM]

Note

Adds a #[heuristic_reordering] join strategy that reorders rule clauses, with full parser/HIR/MIR support, updated index selection, docs, examples, and tests.

• Join Reordering (core)
  • Introduce #[heuristic_reordering] join strategy in rule syntax (ascent_syntax), propagating join_strategy through desugaring, HIR (IrRule) and MIR (MirRule).
  • Implement heuristic clause reordering in MIR: reorder rules/SCCs, recompute indices, validate grounded/used vars, and update dynamic relation indices/maps.
  • Add utilities: CondClause::used_vars, MirBodyItem::used_vars, expose extend_grounded_vars, carry join strategy in summaries.
• Docs/Examples/Tests
  • README: new “Join Reordering” section with example.
  • New example ascent/examples/ascent_heuristic_order.rs demonstrating the attribute.
  • Tests: add test_reordering; minor example tidy-ups.
• BYODS and misc
  • Adjust a BYODS test rule to use let-bindings; add minor allow/typo fixes in related crates.

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