Add policy_hash: String to kb_core::Chunk to align with the §5.5 SQLite
schema (chunks.policy_hash NOT NULL), so kb-store-sqlite persistence is a
straight field copy rather than a recompute.
This is a §9 schema migration:
- §5.5 (the persistence schema) is authoritative.
- §3.5 (the domain model) must accommodate.
The chunker already computed policy_hash for the chunk_id recipe (§4.2);
P1-5 stored it implicitly. We now hold it explicitly on the Chunk so any
DocumentStore::put_chunks impl can read it directly.
Follow-up commits update kb-chunk to populate the field and refresh the
P1-5 snapshot baseline accordingly.
altair823
16b2a5c150