docs: clarify checkpoint integrity wording

Describe SHA256SUMS as pinned rather than cryptographically signed; model objects are unchanged.

README.md

@@ -51,7 +51,7 @@ bash scripts/download_paper_checkpoints.sh waypoint_intact all
 bash scripts/download_paper_checkpoints.sh matrix all
 ```
 
-The wrapper pins `paper-e5-goal-v1`, checks the signed `SHA256SUMS` record,
+The wrapper pins `paper-e5-goal-v1`, checks the pinned `SHA256SUMS` record,
 verifies every downloaded archive, extracts it into `STABLEWM_HOME`, and then
 verifies every `.pt` shard against the checked-in manifest.