Add term identity sidecar

[MesTTo]

Adds a derived term identity sidecar for encoded MORK facts and subterms.

The branch interns complete facts and subterms from encoded paths, records stable TermId and FactId values, caches ground/schematic flags, and exposes read-only counters. The sidecar is derived from PathMap<()> snapshots and does not change the authoritative atom store.

Validation:

• cargo +nightly-2026-06-01 test -p mork --test term_identity 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/term_identity.rs and kernel/tests/term_identity.rs
• jscpd on kernel/src/term_identity.rs and kernel/tests/term_identity.rs
• git diff --check

[MesTTo]

Folding this into #116, which now contains this base commit plus the follow-ups that address the review direction: the structural hash reuses Expr::hash (gxhash128) instead of the hand-rolled FNV here, the PathMap-coupled extend_from_pathmap is gone, and the fact lifecycle rides on top. #116 is mergeable against current main; this branch conflicts. One PR to review instead of a stale base.