feat(session): add explicit leave and immediate slot release

docs/M016_LATENCY_PROCEDURE.md

@@ -14,6 +14,7 @@ browser timestamp from an ESP timestamp.
 | command to painted view | browser | `move` send to next frame after matching view render |
 | handler and send | ESP | existing `/diagnostics` maxima |
 | network round trip | client | ICMP and WebSocket heartbeat samples |
+| ESP heartbeat handling | ESP | bounded acknowledgement preparation interval |
 
 The accepted command acknowledgement contains the authoritative `stateSequence`.
 The browser records a sample only after the complete view with that sequence is
@@ -38,6 +39,10 @@ rendered and the next animation frame occurs. Samples are local-only, capped at
    and send maxima, queue high-water marks, observer view chunk count, and
    ICMP/WebSocket round trips. Attribute a delay only when the matching
    interval is elevated.
+7. For the compact quality indicator, record the rounded `Отклик`, `Связь`,
+   and `ESP` values after the five-second heartbeat cadence has collected a
+   fresh sample. `Связь` is the non-negative round-trip remainder, not a pure
+   Wi-Fi measurement.
 
 ## Safety checks
 

docs/PLAYER_UI.md

@@ -66,6 +66,12 @@ closing this presentation-only menu never sends a command or changes server
 state. Escape and outside activation dismiss it; player actions also close it
 before opening their existing destination.
 
+The header response indicator takes at most one heartbeat sample every five
+seconds, holds one in-flight request, and uses a five-sample median. Good is
+at most 150 ms, delayed is 151--500 ms, and slow is above 500 ms. Hysteresis
+holds good until above 175 ms and slow until at most 450 ms; a sample is stale
+after 15 seconds. Hidden documents and closed sockets do not probe.
+
 The browser stores the direction for each outstanding move request. A matching
 server `blocked` rejection may include only that player's approved breakthrough
 stage, which bends the already rendered bush wall for at most 340 ms without

docs/PROTOCOL.md

@@ -39,6 +39,9 @@ is only echoed for correlation; it grants no authority.
 - `restart`: `{}`. Only connected Player 1 may reset a finished match, and both
   players must be connected. Sessions are preserved while a new authoritative
   maze and game state are created.
+- `leave`: `{}`. Revokes only the authenticated sending session. A player
+  leaving an unfinished match cancels it for all participants; observer and
+  waiting-lobby departures release only that session.
 - `heartbeat`: `{}` after a session has been joined or restored.
 
 ## Server messages
@@ -60,7 +63,10 @@ is only echoed for correlation; it grants no authority.
 - `match_result`: reports the terminal winner or draw.
 - `reset`: reports the authoritative return from a finished match to the
   waiting lobby.
-- `ack`: successful heartbeat correlation.
+- `ack`: successful heartbeat correlation. A heartbeat acknowledgement has
+  `payload.command` set to `heartbeat` and a bounded integer
+  `payload.handlingMs` measured only by the ESP monotonic clock while it
+  prepares that acknowledgement.
 - `error`: a stable English `code` and Russian player-facing `message`.
 
 An observer session can receive snapshots but cannot issue `start`, `move`, or

docs/UI_STYLE.md

@@ -106,6 +106,15 @@ are not MVP requirements.
   help actions during an active player match. The popover is constrained to
   safe areas, closes on outside activation or Escape, and never contains
   primary game, lobby, observer, or result actions.
+- The settings popover shows `Выйти из игры` only for an authenticated session.
+  It opens a confirmation modal; an unfinished player match warns that it ends
+  for everyone, while observer and lobby departures explain that only this
+  browser's place is released.
+- A compact response indicator between the connection state and gear shows a
+  rounded total response time or an explicit unknown, stale, or offline state.
+  Its accessible description and settings details expose `Отклик`, `Связь`,
+  and `ESP`; `Связь` is a transport remainder and does not assign blame to
+  Wi-Fi, the ESP, or a player.
 - Countdown, pause, and result states appear as modal overlays above the map.
 - Textual system notices use one live message rail above the top bar and player
   status area. It shows one message at a time: connection loss has highest