Catalog: a build that learns a platform fetches the firms an older one skipped

A firm on a platform the build didn't know was skipped, yet the cache was
stamped current, so after updating to a build that knows the platform the
sync sent ?since= and the master answered "unchanged": the firm (Vest, on
machines that synced before 96fdd9c) stayed missing until an unrelated
catalog edit. The parser version goes to 3, so those caches fetch in full
once, and a payload now names the platforms it skipped (skippedPlatforms):
a cache whose skipped platform this build knows is no longer current. Kept
rule ids still report from such a cache. scripts/check-catalog.mjs syncs
against a stand-in master on a local port.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/master-protocol.ts

@@ -508,6 +508,9 @@ export interface FirmCatalogPayload {
      *  payload. An older parser dropped fields it didn't know, so a cache
      *  without it, or below the current one, isn't trusted to hold them. */
     parser?: number;
+    /** Instance-local, like parser: the platforms of the firms that parser
+     *  skipped as unknown to it. A build that knows one fetches in full. */
+    skippedPlatforms?: string[];
 }
 
 export interface FirmCatalogUnchanged {