docs(guards): make the layer-dependency baseline an explicit shrink-only ratchet (#6053)

scripts/layer-dependency-baseline.txt

@@ -1,5 +1,11 @@
 # Layer dependency baseline for the rustfs binary crate.
 #
+# RATCHET RULE (backlog#1834): this file only shrinks. A PR may delete lines
+# (after migrating the violation) via --update-baseline; a PR that ADDS a line
+# is baselining a brand-new layering violation and must carry an explicit
+# exemption rationale in its description — the baseline is a migration ledger,
+# not an amnesty list.
+#
 # The guard models production imports as:
 #   composition -> interface -> app -> infra
 #