Add BindingSpace join readiness analysis

[MesTTo]

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