Trace BindingSpace trie traversal

[MesTTo]

Stack dependency: review after #83. This branch is based on the prerequisite head. Until the prerequisite PRs merge, GitHub may include their commits in the aggregate diff; the intended review increment is the new commit at this branch tip.

Traces BindingSpace trie traversal.

The branch adds a non-materializing trace for the trie-backed variable-at-a-time join. trie_join_trace records the variable order, indexed row and trie-node counts, candidate binding count, and each variable-depth domain intersection.

BindingSidecarPlan::explain_selected_trie_trace exposes the trace through the selected-kernel explain path. The trace is present only when the selector chooses the suggested-order trie join.

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 binding_space 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 and kernel/src/binding_space.rs
• jscpd on kernel/src/binding_plan.rs, kernel/src/binding_space.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

[MesTTo]

Closing with the rest of the binding_space line: these counters instrument the sidecar's reimplemented join, which is parked, so the numbers they report are not the engine's. The concept is wanted (traversal statistics under a flag, e.g. every-Nth-point and average path length) and should be rebuilt as a small standalone PR instrumenting the real PathMap/ProductZipper traversal.