Add BindingSpace join readiness analysis

[MesTTo]

Stack dependency: review after #74. 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 root-domain analysis for BindingSpace sidecar plans.

The branch factors plan execution so arrangement-backed relations can be opened once, inspected, and then joined. It adds BindingSidecarAnalysis, per-variable root-domain counters, and a suggested variable order that prefers connected variables with smaller visible domains. The regression checks the suggested order for the transitive edge plan.

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/arrangements.rs, kernel/src/binding_space.rs, and kernel/src/test_sidecar_queries.rs
• git diff --check
• normal diffstat matched git diff -w --stat

[MesTTo]

Closing: part of the un-wired binding_space sidecar line that reimplements the kernel's ProductZipper join (relational matcher / arrangements / trie-join / expression-trie) over a reinvented term_identity::TermId(u64), instead of reusing Expr and PathMap. The kernel core (space.rs/main.rs/sinks.rs/sources.rs/pure.rs) references none of these modules, so nothing is wired to the MeTTa surface. This is the 'reimplements rather than reuses / un-wired feature island' concern. Parking the line; if a specific piece is wanted (e.g. a new PathMap zipper type with property tests against existing ops), it can be reopened as a small standalone PR on PathMap primitives.