feat: let a profile say its pin is held on purpose

nethersx2-turnip-classic documents the shim rules of the revision it
pins, and says so in prose: the classic build is that revision. Compared
to HEAD it reported the same two changes every pass, and recaling would
have repointed its refs at rules this build never had.

pin_frozen says the freeze is deliberate, and the profile is then judged
against its own revision, exactly as one pinned to a superseded tag
already was. Recale and bump are refused for it, which is what makes the
declaration safe to trust.

schemas/emulator.schema.json

@@ -598,6 +598,10 @@
       "type": "string",
       "description": "Branch the port was read from, when it is not the default one."
     },
+    "pin_frozen": {
+      "type": "string",
+      "description": "Why source_commit is held on purpose rather than followed. The profile is then judged against its own revision, like one pinned to a superseded tag: comparing it to HEAD says nothing and recaling would repoint its refs at code the build it documents never had."
+    },
     "upstream_gone": {
       "type": "string",
       "description": "Why the declared upstream can no longer be reached, when that is settled rather than transient. Records the death instead of leaving the profile failing every pass; profile_sync reports the contradiction if the forge answers again."