p1-5: scaffold kb-chunk crate with MdHeadingV1Chunker skeleton

Adds the new workspace member with the bare Chunker impl shape:
chunker_version() returns "md-heading-v1"; policy_hash() blake3-hashes
canonical JSON of ChunkPolicy and truncates to 16 hex chars; chunk()
is an empty stub the next commits fill in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -539,6 +539,23 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "kb-chunk"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "blake3",
+ "kb-config",
+ "kb-core",
+ "kb-normalize",
+ "kb-parse-md",
+ "serde",
+ "serde_json",
+ "serde_json_canonicalizer",
+ "time",
+ "tracing",
+]
+
 [[package]]
 name = "kb-cli"
 version = "0.1.0"