Stage 4: lobby & social (matchmaking, friends, blocks, chat+nudge, invitations, profile, email, multi-player drop-out)

Engine: multi-player drop-out-and-continue with a per-game tile disposition (remove default / return), resigned seats skipped and excluded from the win, leaver rack never revealed; 2-player behaviour unchanged.

New domains (service/store, no HTTP yet): internal/social (friend request/accept graph, per-user blocks, per-game chat with nudge as a message kind, content filter via mvdan.cc/xurls/v2 + leet/separator normaliser + phone heuristic) and internal/lobby (in-memory variant-keyed matchmaking pool, friend-game invitations invite->accept with lazy 7-day expiry). account gains profile editing and the email confirm-code flow (Mailer seam: SMTP or log mailer).

Migration 00003_social.sql + regenerated jet. main wires the new services into the server (accessors for the Stage 6 handlers); robot substitution stays in Stage 5, REST/stream/push in Stage 6/8. Docs (PLAN, ARCHITECTURE, FUNCTIONAL+ru, TESTING, README) updated.

docs/TESTING.md

@@ -33,7 +33,20 @@ tests or touching CI.
   end, **journal-replay equivalence**, the turn-timeout sweep with away-window
   grace, resign win/loss and statistics, the hint allowance-then-wallet policy,
   word-check and complaint capture, and per-game-lock serialisation). The robot
-  balance/margin regression tests arrive with Stage 5.
+  balance/margin regression tests arrive with Stage 5. Stage 4 adds the engine's
+  **multi-player drop-out** cases (continue after one resign, last-survivor win,
+  the tile-disposition bag effect) and a domain integration test for a 3-player
+  **timeout that continues**.
+- **Social & lobby** *(Stage 4+)* — `backend/internal/social` unit-tests the chat
+  **content filter** (links/emails/phones plus obfuscated forms) and
+  `backend/internal/lobby` unit-tests the in-memory **matchmaker** (FIFO pairing,
+  cancel, per-variant pools) with a fake game creator. Postgres-backed `inttest`
+  covers the friend request/accept lifecycle with the block/toggle guards, the
+  per-user block (and its severing of friendships), chat post/list with the IP,
+  content and block-visibility rules, the nudge turn/rate-limit rules, the
+  invitation flow (all-accept starts the game, decline cancels, lazy expiry,
+  inviter-only cancel), and the email confirm-code flow (request/confirm, taken
+  email, expiry and attempt-cap) with a fixture mailer.
 
 ## Principles