docs(events): a step's retry delay, MODULE_API 1.15.0 (PLAN_REDESIGNS §14.3.15)

- MODULE_API.md: 1.15.0, the failure envelope's retryAfterMs (clamped to
  one to thirty minutes, one attempt, ignored beside retry: false), the
  rule bullet and the registration line.
- EVENTS.md §F: the envelope rule and the flat delay as a default.
- STEAM_SIGNIN.md: its module contract moves to 1.16.0.
- PLAN_REDESIGNS: §14.3.15, the stage-2 record (six settled points, the
  test-harness gap found on the way), and pointers from §9's summary and
  §14.3.13.

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

website/STEAM_SIGNIN.md

@@ -60,7 +60,8 @@ assertion is single-use, which the existing single-use tx cookie already enforce
 - Admin → Auth shows Steam with an enable switch and, per Q3, an optional Web API key stored like a client secret.
 - The account page lists Steam identities with a *Link Steam* button, and each linked Steam account is shown with its
   id (or name, Q3) and an *Unlink* button.
-- **Module contract, MODULE_API 1.15.0 (additions only):** a read of a user's verified identities for one provider,
+- **Module contract, MODULE_API 1.16.0 (additions only; 1.15.0 was taken by a step's `retryAfterMs`, Rust
+  PLAN_REDESIGNS §14.3.13):** a read of a user's verified identities for one provider,
   and a hook when one is linked or unlinked. Names are settled at build. Module-uo uses neither, and runs its usual
   frozen-manifest proof against the new core.
 - OpenAPI annotations and the regenerated spec for anything new or changed; `BACKEND_DESIGN.md` §3/§4/§6.