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/index.js

@@ -158,6 +158,16 @@ module.exports = function register(ctx, api) {
   api.onBoot(boot.onBoot)
   api.onShutdown(boot.onShutdown)
 
+  // Sign in with Steam (MODULE_API 1.17.0; PLAN_REDESIGNS §15). A Steam account
+  // linked on the site is a linked Rust player (D392): core asks before it writes
+  // the sign-in identity, and this module refuses a Steam id that is another
+  // account's Rust link (D396); once it is written, the Rust link is made. Both
+  // are in the links model beside `/link`'s own write, and neither runs at
+  // registration — core calls them from a signed-in user's link flow.
+  const links = require('./model/links/links.model') // eslint-disable-line global-require
+  api.onIdentityLinking(links.checkSteamSignin)
+  api.onIdentityLinked(links.linkFromSteamSignin)
+
   // The leases (PLAN.md §27, protocol 8): what an event may BORROW on a server
   // and must give back. Core's `core.lease` is the verb; these are the values it
   // may name and the four callables each ships. Every lease is targeted and the