Add stable checkpoint decision diagnostics

docs/internal/state.md

@@ -43,6 +43,17 @@ not permit stale downstream reuse. The CLI's selective recomputation policy
 records `forced_recompute` for the selected step and its dependents while
 requiring compatible predecessor checkpoints.
 
+The loader assigns a typed category and reason code at each validation site;
+diagnostic prose is not classified after the fact. The runner then applies
+forced-execution policy, validates reusable artifact bytes through the prepared
+codec, and records the final decision before enforcing a required-predecessor
+failure. That failure names only the step, lane, and stable reason code. Decision
+detail passes through one UTF-8-safe bounded sanitizer and contains only
+allowlisted diagnostic context, never payloads, references, credentials,
+environment values, or physical paths. Typed categories and codes remain intact
+through pipeline events and become strings only in manifest and debug-summary
+JSON.
+
 `internal/core/fileio` provides confined atomic file writes used by state
 collaborators. The chunk-plan store retains its stronger entry validation.