Add expression trie BindingSpace access

[MesTTo]

Stack dependency: review after #77 and #78. 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 expression-trie access for BindingSpace plans.

The branch adds a BindingAccessPlan::Pattern access path. A pattern factor builds a typed expression trie from the current sidecar snapshot, retrieves candidate facts by grounded prefix, exact-filters those candidates, and projects matched user slots into a BindingRelation. It also adds PatternProjection validation and counters for indexed facts, candidate facts, exact pattern checks, and matches.

Review this after both pr/mork-binding-domain-cursor and pr/mork-expression-trie-sidecar. The stack base combines those prerequisites so this PR diff is only the BindingSpace integration layer.

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 expression_trie 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/test_sidecar_queries.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: 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.