feat(rust): a Steam account linked on the site is a Rust link (D392, D396)

Registers core's MODULE_API 1.17.0 identity hooks (coreApi ^1.17.0):
onIdentityLinking refuses a Steam id that is another website account's Rust
link (D396); onIdentityLinked writes the Rust link with the PRIMARY KEY as the
race guard, reconciles Teams and raises the linked notice, as /link does.
Unlinking Steam leaves the Rust link (D393); the Rust account page says so.

ci/core-ref.json still pins the 1.15.0 core and must move to the website
branch head carrying 1.17.0 before CI can pass.

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

server/test/_fakes.js

@@ -166,6 +166,9 @@ function fakeApi() {
     registerSlashCommands(commands) { once('registerSlashCommands'); record.slashCommands = commands },
     onBoot(fn) { once('onBoot'); record.hooks.onBoot = fn },
     onShutdown(fn) { once('onShutdown'); record.hooks.onShutdown = fn },
+    // MODULE_API 1.17.0 (Steam sign-in, PLAN_REDESIGNS §15).
+    onIdentityLinking(fn) { once('onIdentityLinking'); record.hooks.onIdentityLinking = fn },
+    onIdentityLinked(fn) { once('onIdentityLinked'); record.hooks.onIdentityLinked = fn },
   }
   api.record = record
   return api