fix(corpus): harden canonical chunk and frontmatter invariants

.superpowers/sdd/evidence-task-3-report.md

@@ -37,3 +37,23 @@
   configuration before ingestion is wired.
 - Character limits use Python Unicode code points (`len`), not UTF-8 bytes or tokenizer tokens;
   this is recorded in the chunk-policy metadata and tested with non-ASCII content.
+
+## Review hardening follow-up
+
+- Chunk IDs now bind the canonical document identity, document content hash, ordinal, chunk hash,
+  and a canonical SHA-256 fingerprint of every `ChunkPolicy` field. Identical content in separate
+  documents and same-version policies with different limits cannot collide.
+- Boundary-aware slicing now retains separators in the slices. Concatenating every chunk exactly
+  reconstructs the canonical document for repeated spaces, tabs, blank lines, Markdown hard
+  breaks, fenced code, whitespace-only input, Unicode, and overlong tokens; every slice remains
+  within `max_chars`.
+- Frontmatter uses a bounded `SafeLoader` variant: duplicate keys, anchors/aliases, structures
+  deeper than 20 nodes, and documents larger than 1000 composed nodes are rejected. YAML parse,
+  JSON type, credential-safety, and resulting canonical-model errors attributable to frontmatter
+  map to `PermanentNormalizationError(reason="invalid_frontmatter")`; invalid pipeline policy
+  remains a programmer-facing `ValueError`.
+- Follow-up TDD evidence: the expanded focused suite first reported 11 expected failures against
+  the prior implementation, then passed **45/45** across normalization, chunking, and manifest
+  invariants.
+- Follow-up full verification: **586 passed, 5 deselected**. Targeted Ruff is clean. Full Ruff
+  continues to report the same **34 unrelated pre-existing** violations in legacy tests.