v0.5.0 — multiplayer Phases 2 and 3: a server that runs a game and survives being restarted

Phases 0-1 shipped in v0.4.0 (seat/identity split, per-player turn state, the Session boundary).
This lands Phase 2 (server core, one game, no lobby) and Phase 3 (persistence and resumption) per
docs/architecture/multiplayer.md §12. Phases 4-6 (lobby/reconnection, the 22 opponent-directed
cards, StartOS packaging) are still ahead.

Phase 2: src/server/session.ts hosts a game in pure logic (no sockets) on top of game.ts's existing
Game/submit/currentActor/actionMenu; it verifies seat === currentActor(game) itself before calling
submit, since submit() trusts its caller and a server can't. src/server/http.ts and index.ts add
POST /api/game, GET /api/stream (SSE, per-seat), POST /api/intent, and static serving of dist/.
src/sim/frame-delta.ts is a purpose-built per-seat board delta for one live push at a time. Found
and fixed along the way: actionMenu(game, seat) only used seat for the hand field, so a server
computing every connected seat's Menu would have handed the acting player's legal moves to a
waiting seat. Verified with a live end-to-end smoke test (2-player game, two SSE streams, a
rejected intent from the wrong seat, an idempotent resend) plus test/server/session.test.ts and
test/redaction.test.ts. Not verified: an actual browser (none available in this environment).

Phase 3: src/server/persistence.ts writes game.json and turn-timings.json, atomic-rewrite-then-
rename. game.ts gained fromMultiplayerSave, fixing a narration-attribution bug found while testing
it (fromSave's replay loop drops the actor argument, invisible in solitaire, unreadable the moment
there's more than one seat — fromSave itself still has this gap, deliberately untouched). Verified
live: server killed and restarted mid-game, both seats reconnected exactly where they left off.

Two rules bugs found while building this: the New Train phase never implemented its car-placement
round (every car of every train was placed by the Superintendent alone, in every mode, all along —
now reads the round position off tray.consist.length); and victory conditions are now one shared,
configurable GameConfig set across solitaire/competitive/coop instead of a fixed length lookup and
a dead firstToTarget condition.

Also folds in the three fixes already released on the patch line as v0.4.9b/c/d: a switching
train's crew badge failing to draw once it left the Office square, an unload that always took the
westmost car regardless of which was picked, and a legal decision that could render with zero
buttons.

docs/testing/0.5.0-test-plan.md and three reported-bug save files (docs/station-master-seed*.json)
included for reproducibility. tools/jitsi-harness/ deliberately left untracked — unrelated
side-project work, not part of this release. 635 tests, 0 failures.

test/frame-delta.test.ts

@@ -0,0 +1,87 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+
+import { areaOf } from '../src/engine/apply.ts';
+import { createGame } from '../src/engine/setup.ts';
+import { coordKey } from '../src/engine/state.ts';
+import type { GameConfig, GameState } from '../src/engine/state.ts';
+import { applyDelta, deltaFrame } from '../src/sim/frame-delta.ts';
+import { snapshot } from '../src/sim/view.ts';
+
+const config: GameConfig = {
+  mode: 'solitaire',
+  days: 5,
+  minCombinedRevenue: 0,
+  maxCollisionsPerDay: 0,
+  maxCollisionsTotal: 0,
+  pvpCardsAllowed: false,
+  optionalRules: {
+    reducedVisibility: false,
+    sisterTrains: false,
+    employeeRotation: false,
+    emergencyToolbox: false,
+  },
+};
+
+const game = (seed = 1): GameState => createGame({ id: 'g', seed, config, playerNames: ['a'] });
+
+describe('deltaFrame — the live one-step-back board diff', () => {
+  it('nulls cells/facilities/division when the board has not changed, keeps everything else', () => {
+    const s = game();
+    const first = snapshot(s, [], null);
+    s.players[0]!.revenue = 42; // a non-board change
+    const second = snapshot(s, [], null);
+
+    const delta = deltaFrame(first, second);
+    assert.equal(delta.cells, null, 'unchanged cells were not nulled');
+    assert.equal(delta.facilities, null, 'unchanged facilities were not nulled');
+    assert.equal(delta.division, null, 'unchanged division was not nulled');
+    assert.equal(delta.revenue, 42, 'a real change was dropped along with the unchanged board');
+  });
+
+  it('sends the real board when it changed', () => {
+    const s = game();
+    const first = snapshot(s, [], null);
+    const area = areaOf(s, 0);
+    area.grid.set(coordKey({ row: area.runningRow - 1, col: 0 }), {
+      geometry: { kind: 'track', geometry: 'straight' },
+      baseOperationalRail: true, standing: [], standingWest: 0, facility: null, modifiers: [], enhancements: [],
+    });
+    const second = snapshot(s, [], null);
+
+    const delta = deltaFrame(first, second);
+    assert.notEqual(delta.cells, null, 'a real board change was nulled');
+    assert.deepEqual(delta.cells, second.cells);
+  });
+
+  it('never nulls anything on a first connect (no previous Frame)', () => {
+    const s = game();
+    const frame = snapshot(s, [], null);
+    const delta = deltaFrame(null, frame);
+    assert.deepEqual(delta.cells, frame.cells);
+    assert.deepEqual(delta.facilities, frame.facilities);
+    assert.deepEqual(delta.division, frame.division);
+  });
+
+  it('applyDelta round-trips: merge(delta(A, B)) against A reconstructs B exactly', () => {
+    const s = game();
+    const a = snapshot(s, [], null);
+    s.players[0]!.revenue = 7;
+    const area = areaOf(s, 0);
+    area.grid.set(coordKey({ row: area.runningRow - 1, col: 1 }), {
+      geometry: { kind: 'track', geometry: 'straight' },
+      baseOperationalRail: true, standing: [], standingWest: 0, facility: null, modifiers: [], enhancements: [],
+    });
+    const b = snapshot(s, [], null);
+
+    const merged = applyDelta(a, deltaFrame(a, b));
+    assert.deepEqual(merged, b);
+  });
+
+  it('throws rather than silently reconstructing garbage when previous is missing but the delta says "unchanged"', () => {
+    const s = game();
+    const frame = snapshot(s, [], null);
+    const delta = deltaFrame(frame, frame); // everything nulled, since nothing changed
+    assert.throws(() => applyDelta(null, delta));
+  });
+});