Reuse expression trie across pattern factors

[MesTTo]

Reuses one expression-trie index across pattern factors in a BindingSpace plan.

The branch adds a per-execution open context for BindingAccessPlan factors. Pattern factors now share one typed expression-trie build for the sidecar snapshot instead of rebuilding the same index for each factor. It also records expression_trie_builds so tests can assert the shared-index path directly.

Review this after pr/mork-expression-trie-binding-access. Against that branch, the diff is the open context, the build counter, and one regression covering two pattern factors.

Validation:

• cargo +nightly-2026-06-01 test -p mork --lib binding_plan with RUSTFLAGS='-C target-cpu=native'
• cargo +nightly-2026-06-01 test -p mork --lib with RUSTFLAGS='-C target-cpu=native'
• rustfmt --check on kernel/src/binding_plan.rs
• jscpd on kernel/src/binding_plan.rs, kernel/src/expression_trie.rs, kernel/src/pattern_relations.rs, kernel/src/test_exprs.rs, and kernel/src/test_sidecar_queries.rs
• git diff --check
• normal diffstat matched git diff -w --stat