kb-core: add policy_hash field to Chunk struct (P1-6 schema reconcile)

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.

crates/kb-core/src/chunk.rs

@@ -6,6 +6,12 @@ use crate::document::SourceSpan;
 use crate::ids::{BlockId, ChunkId, DocumentId};
 use crate::versions::ChunkerVersion;
 
+/// A unit of retrievable text per design §3.5 + §5.5.
+///
+/// `policy_hash` is the chunker's hex digest of the active `ChunkPolicy`
+/// (e.g. `target_tokens`, `overlap_tokens`). It mirrors the §5.5 SQLite
+/// schema column so persistence is a straight copy, and feeds the
+/// `chunk_id` recipe (§4.2) so policy edits invalidate downstream IDs.
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Chunk {
     pub chunk_id: ChunkId,
@@ -16,4 +22,5 @@ pub struct Chunk {
     pub source_spans: Vec<SourceSpan>,
     pub token_estimate: usize,
     pub chunker_version: ChunkerVersion,
+    pub policy_hash: String,
 }