Explain BindingSpace sidecar execution choices

[MesTTo]

Stack dependency: review after #82. 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.

Explains BindingSpace sidecar execution choices.

The branch adds BindingSidecarExecutionReport and BindingSidecarPlan::explain_execution. The report opens the sidecar factors, returns the root-domain analysis, and returns the selected execution choice without running the final join.

choose_execution and execute_selected now use the same explain path, so the reported choice and the executed choice stay aligned.

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

[MesTTo]

Closing: this is part of the un-wired binding_space sidecar that reimplements the kernel's ProductZipper join (generic_join/trie_join/leapfrog) over a parallel term identity instead of reusing Expr/PathMap, and it is not wired to the MeTTa surface. Cost-based execution-kernel selection is also out of scope for the kernel. Parking the whole execution-kernel line. Narrates the parked execution-kernel selector.