docs(website): Steam sign-in built (MODULE_API 1.17.0)

STEAM_SIGNIN.md gains its Built section (section 8) and status;
MODULE_API.md records 1.17.0 (ctx.identities.list, onIdentityLinking,
onIdentityLinked); BACKEND_DESIGN.md gains the auth_providers /
user_identities shapes, the identities route with ?id=, and the Steam
security notes in section 6; PLAN_REDESIGNS section 15 notes the build.

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

modules/rust/PLAN_REDESIGNS.md

@@ -3673,6 +3673,11 @@ What this module takes from it:
 - When it lands, Module-Rust's account screens and the link flow are updated in their own PR. Rust's grants
   already follow the website user to every linked Steam id (D28), so nothing else changes.
 
+**Built 2026-10-19** on `edge` with the website and Android-app halves (`website/STEAM_SIGNIN.md` §8):
+`checkSteamSignin` and `linkFromSteamSignin` in the links model, registered as core's
+`onIdentityLinking` / `onIdentityLinked`, `coreApi ^1.17.0`. The live walk with a real Steam account is
+owed with the end walk's in-game rows.
+
 ---
 
 ## 16. Console commands as event steps and rewards (D382–D384)