Expose BindingSpace domain cursor

[MesTTo]

Exposes a BindingSpace domain cursor contract.

The branch adds BindingDomainCursor, an in-memory slice cursor adapter for relation-trie domains, and cursor-level counters on TrieJoinStats. The trie join now routes domain intersection through the cursor contract, and the tests cover monotone seek, next, key, and end behavior.

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