Add relational pattern matcher

[MesTTo]

Stack dependency: review after #69. 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 relationalized pattern lowering and exact sidecar matching over canonical term identities.

The branch lowers encoded patterns into App_n(parent, children...) atoms, tracks user-visible variable slots, and matches complete fact roots by TermId equality. It compares the sidecar matcher against the existing product-query path for a repeated-variable pattern. This branch is stacked on the term-identity pattern helper branch.

Validation:

• cargo +nightly-2026-06-01 test -p mork --lib pattern_relations 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/pattern_relations.rs
• jscpd on kernel/src/pattern_relations.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.