Expose pattern term identity helpers

[MesTTo]

Adds term identity helpers needed by derived pattern indexes.

The branch adds insert_term for interning pattern terms without creating fact records, exposes facts() for read-only fact iteration, and factors complete-term parsing through one internal helper so insert_term and insert_fact share trailing-byte and rollback behavior.

Validation:

• cargo +nightly-2026-06-01 test -p mork --lib term_identity with RUSTFLAGS='-C target-cpu=native'
• rustfmt --check on kernel/src/term_identity.rs
• jscpd on kernel/src/term_identity.rs
• git diff --check
• normal diffstat matched git diff -w --stat