Trace BindingSpace trie factor participation

[MesTTo]

Stack dependency: review after #84. 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.

Adds factor participation metadata to BindingSpace trie traces.

The branch records which relation factors participate at each variable-depth trace step and the domain length contributed by each participating relation. The existing trace still records the surviving intersection and cursor counters.

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: this trace-replay/diff subsystem goes well beyond the simple traversal statistics the kernel would want under a flag, and it instruments the un-wired binding_space join reimplementation rather than the real PathMap/ProductZipper traversal. Parking it (a minimal traversal-counter, pointed at the real zipper, can be revisited separately). Folds into #84.