Refactor: split oversized modules + dedup triplicated helpers

[thorwhalen]

Maintenance / AI-context hotspots (raw LOC): trans.py 3492, signatures.py 5403, caching.py 2675, paths.py 2270, util.py 2356. Hard to navigate; expensive to load into an agent's context.

Proposal (incremental, non-breaking — keep re-exports):

• trans.py: extract codec definitions → kv_codecs; path-addons/aliases → a store_addons module.
• signatures.py / caching.py: carve cohesive sub-modules.

Also dedup (same hygiene pass):

• identity / identity_func defined ~3× across modules.
• HashableMixin / HashableDict duplicated across util / caching / mixins.

See misc/docs/dol_architecture_map.md §11 (ranked tech-debt).