Add expression trie sidecar

[MesTTo]

Adds a derived typed expression-trie sidecar over canonical term roots.

The branch builds a snapshot-local trie from complete fact roots, extracts the grounded prefix of a pattern, returns candidate fact IDs below that prefix, and exact-filters those candidates through the relational pattern matcher. It also factors the repeated pattern fixture used by the sidecar tests so the new tests do not duplicate existing setup code.

Validation:

• 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/expression_trie.rs, kernel/src/pattern_relations.rs, and kernel/src/test_exprs.rs
• jscpd on kernel/src/expression_trie.rs, kernel/src/pattern_relations.rs, and kernel/src/test_exprs.rs
• git diff --check
• normal diffstat matched git diff -w --stat