kb-chunk: populate Chunk.policy_hash field

Set the new policy_hash field on every emitted Chunk to the same hex
already computed for the chunk_id recipe (§4.2). No recipe / chunk_id
change — only the field on the struct is now populated.

Pairs with the kb-core hotfix (preceding commit) and unblocks P1-6's
DocumentStore::put_chunks to read chunk.policy_hash directly per §5.5.

crates/kb-chunk/src/md_heading_v1.rs

@@ -355,6 +355,7 @@ fn build_chunk(
         source_spans,
         token_estimate,
         chunker_version: chunker_version.clone(),
+        policy_hash: policy_hash.to_string(),
     }
 }