Summarize BindingSpace trie trace replay impact

[MesTTo]

Stack dependency: review after #87. 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.

Summarizes BindingSpace trie trace replay impact.

The branch adds TrieJoinTraceReplayShape::impact_for_bound_prefix. It reports the trace-local work below an exact bound-prefix context, including covered step indexes, step count, candidate bindings, participating domain sources, domain values, surviving intersection values, and maximum replay depth.

Validation:

• 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_space.rs
• jscpd on kernel/src/binding_space.rs, 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

[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).