docs(offline): document hotseat pass-and-play

ARCHITECTURE (offline section) + FUNCTIONAL (+_ru mirror): the local
pass-and-play mode — host/referee master PIN, per-seat optional PIN
locks (social lock, salted SHA-256, client-only), the board-visible seat
unlock, host overrides (skip/exclude/terminate), no hints/chat, and the
master-PIN-gated lobby deletion.

docs/ARCHITECTURE.md

@@ -1251,8 +1251,22 @@ toggle — distinct from the transient gateway-reachability signal — that tint
 an *Offline* chip and confines play to on-device `vs_ai` games. The **offline lobby lists only those
 device-local games** (reconstructed by replaying the IndexedDB move journal) and its New-vs-AI entry
 creates one through the in-browser engine — the same game screen then drives it, the robot replying
-locally; online-only affordances (the Stats tab, the friend/random options in New Game) are disabled
-or hidden. A `vs_ai` hint (online and offline alike) is unlimited and wallet-free but idle-gated
+locally; online-only affordances (the Stats tab, the random-opponent option) are disabled
+or hidden, and New Game's *with friends* becomes the entry to **local pass-and-play (hotseat)**.
+A hotseat game is a device-local **2-4 human** game built through the same engine (`hotseat` +
+per-seat/host PIN locks on the record; the seats carry names but no accounts). A **mandatory host
+(referee) PIN** gates the roster at creation and, in-game, the referee overrides — **skip** the
+current turn, **exclude** a player (a forced seat resign, `resignSeat`) or **terminate** the game
+(deleted, no winner/loser); each seat may also carry its own **optional PIN** that withholds its rack
+(the board stays visible — only the tray is gated) until it is entered, so players pass the device
+around freely and lock a seat only against a distrusted tablemate. The unlock is **per turn**
+(re-locks on advance). PINs are a **social lock, not cryptography** — a salted SHA-256 kept only in
+the device record (`lib/pin`); a 4-digit PIN is trivially brute-forceable and the racks live in
+client memory regardless, so they defeat a casual glance, not a determined peek. A hotseat game is
+**not `vs_ai`**: hints (the freed control becomes the host button) and chat/self-resign are gone, and
+its lobby card — active *and* finished — is master-PIN-gated to delete (so the last mover cannot
+instantly wipe it); a naturally finished game is saved with its result like any local game. A `vs_ai`
+hint (online and offline alike) is unlimited and wallet-free but idle-gated
 (unlocked ~30 min into a stuck turn), and counts toward no hint statistic. The **source** reports the
 **seconds left** (`hint_unlock_left_seconds` on the game view) — computed by the **backend from the
 server clock** online (which also enforces the gate, returning `hint_locked` for an early request),