Add briscola: classic two-player and briscola a quattro

New tavolo-briscola distribution implementing briscola on the platform:

- pure rules (state.py, engine.py): 3-card deals, a face-up trump that
  is the last card drawn, trump-first trick resolution (A > 3 > K > H >
  J > 7 > 6 > 5 > 4 > 2), 120 points per deal, 60-60 patta re-deals, a
  configurable target score (default 61), deal-end acknowledgements and
  turn auto-play;
- BriscolaEngine (briscola, 2 players) and BriscolaQuattroEngine
  (briscola_a_quattro, 4 players in fixed partnerships) adapters;
- registered in tavolo-app, packaged in the Docker image, documented.

Platform fixes surfaced by a two-player lobby:

- the websocket state and snapshot payloads now carry seats_open (they
  were only in the REST lobby payload), so lobbies with player counts
  other than four render correctly; seats_open is clamped at zero;
- the web client logs unparseable server messages instead of silently
  dropping them.

Web client: briscola table for both variants (stock, face-up trump,
trick, last-trick banner), deal summary modal and game-over overlay.

Tests: tavolo-briscola rules/adapter suites, app integration tests
(lobby, websocket play, turn timeout, deal-end acks, persistence), a
platform seats_open test and frontend model parsing tests.

server/packages/tavolo-briscola/README.md

@@ -0,0 +1,58 @@
+# tavolo-briscola
+
+Briscola — the Italian trick-taking game — as a
+[`tavolo-platform`](../tavolo-platform/README.md) game implementation.
+Two variants share one rules core:
+
+- `briscola` — the classic two-player game;
+- `briscola_a_quattro` — four-player fixed-partnership briscola.
+
+## Contents
+
+- `state.py` — `BriscolaState` (pure game data: phases, players, hands,
+  stock, face-up briscola, tricks, scores, deadlines), `PlayerState`,
+  `Card`, `TrickCard`, `Trick`, with JSON (de)serialization. No session
+  envelope, no transport, no I/O.
+- `engine.py` — the pure rules engine: deck, deals, trick resolution
+  (trump first, then the led suit; strength ace > three > king > horse >
+  jack > 7 > 6 > 5 > 4 > 2), the draw order (trick winner first, the
+  face-up briscola last), card points (ace 11, three 10, king 4, horse 3,
+  jack 2), patta re-deals and the target-score match end, deal-end
+  acknowledgements and auto-play. Deterministic and I/O-free apart from
+  logging, so the whole rule set is unit-testable.
+- `errors.py` — briscola-specific errors (`CardNotInHand`); every other
+  failure mode is a shared `tavolo.platform.errors` subclass.
+- `plugin.py` — `BriscolaEngine` and `BriscolaQuattroEngine`
+  (`GameEngine`): the platform-facing adapters. They validate the
+  `target_score` creation option (61–120, default 61) and extract the
+  `MatchResult` (teams, winner, per-player scores and the match summary
+  persisted as the match's JSON `result`).
+
+Timeouts are constructor arguments (`turn_timeout_seconds`,
+`hand_ack_timeout_seconds`), wired from the environment by the
+application composition root.
+
+## Rules
+
+- Three cards per player; the next card is turned face up: its suit is
+  the trump (briscola) for the deal and it is the last card drawn.
+- The winner of a trick is the highest trump, or the highest card of the
+  led suit when no trump was played. The winner takes the trick and leads
+  the next one.
+- After each trick everyone draws back up to three cards, starting with
+  the trick winner.
+- Card points total 120 per deal; a 60–60 deal is a *patta*, its points
+  are discarded and the deal is replayed. The first team to the target
+  score wins; a tie at or above the target is broken by another deal.
+- Between deals of an unfinished match every player acknowledges the
+  summary (or the hand-end timeout deals on), exactly like scopone.
+
+## Development (from `server/`)
+
+```sh
+.venv/bin/python -m unittest discover -s packages/tavolo-briscola/tests
+.venv/bin/python -m mypy -p tavolo.briscola
+```
+
+`test_engine.py` covers the pure rules; `test_plugin.py` covers the
+platform contract (actions, deadlines, serialization, results).