kb-chunk: regenerate long-section.chunks.snapshot.json baseline

The snapshot now includes the policy_hash field on every Chunk per the
preceding kb-core schema change. chunk_ids are unchanged because the
chunk_id recipe (§4.2) already incorporated policy_hash via the chunker —
the field is simply now visible in the wire form.

Regenerated via:
  UPDATE_SNAPSHOTS=1 cargo test -p kb-chunk long_section_chunks_snapshot

fixtures/markdown/long-section.chunks.snapshot.json

@@ -11,6 +11,7 @@
     "heading_path": [
       "Alpha"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 1,
@@ -43,6 +44,7 @@
       "Alpha",
       "Alpha Sub"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 7,
@@ -69,6 +71,7 @@
       "Alpha",
       "Alpha Sub"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 53,
@@ -90,6 +93,7 @@
     "heading_path": [
       "Beta"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 55,
@@ -115,6 +119,7 @@
     "heading_path": [
       "Beta"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 64,
@@ -135,6 +140,7 @@
     "heading_path": [
       "Beta"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 66,
@@ -157,6 +163,7 @@
     "heading_path": [
       "Gamma"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 68,
@@ -188,6 +195,7 @@
     "heading_path": [
       "Gamma"
     ],
+    "policy_hash": "de6868f3b7949242",
     "source_spans": [
       {
         "end": 72,