feat(links): unlink every Steam id before an account is deleted (MODULE_API 1.20.0)

ACCOUNT_DELETION.md §4.4 (AD10). api.onUserDeleting calls links.unlinkForDeletion,
which runs the player's own unlinkOwned per link, logs rust.account.unlink with
reason 'account deleted', and marks every server's permission sync dirty so the
withdrawn grants reach the game on the next tick. coreApi ^1.20.0; core-ref pinned
to website#232's merge (7bd93b8); engagement manifest regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/test/_fakes.js

@@ -173,6 +173,8 @@ function fakeApi() {
     onIdentityLinked(fn) { once('onIdentityLinked'); record.hooks.onIdentityLinked = fn },
     // MODULE_API 1.18.0 (the public demo's seed, DM15).
     onDemoSeed(fn) { once('onDemoSeed'); record.hooks.onDemoSeed = fn },
+    // MODULE_API 1.20.0 (account deletion, ACCOUNT_DELETION.md §4.4).
+    onUserDeleting(fn) { once('onUserDeleting'); record.hooks.onUserDeleting = fn },
   }
   api.record = record
   return api