Select BindingSpace sidecar execution kernel

[MesTTo]

Stack dependency: review after #81. 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.

Selects a BindingSpace sidecar execution kernel.

The branch adds a conservative execution choice API for BindingSidecarPlan. Single-factor, disconnected, and small explicit-input plans stay on the reference Generic Join path. Plans with shared-variable pruning and enough projected rows use the suggested-order trie join.

The selected execution result reports the chosen kernel, reason, variable order, projected row count, shared-variable count, and optional trie 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 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.