feat(ui): tile-crossword loading splash for cold lobby open

On a cold app open the lobby's game list arrives over the network; on a
slow link the empty "no games yet" line flashed before the games loaded.
Add a full-screen tile splash that lays a Scrabble crossword of ЭРУДИТ /
ЗАГРУЗКА / ОЖИДАНИЕ (Эрудит point values, hardcoded since the alphabet
table is not cached at boot) until the lobby's first load settles, then
removes itself to reveal the populated list.

- lib/splash.ts: pure layout + reveal schedule (unit-tested).
- components/Splash.svelte: App-level overlay; per-tile drop-in; loops
  ЗАГРУЗКА → ОЖИДАНИЕ until ready, dismisses on a word boundary. Static
  ЭРУДИТ under reduced motion / the mock build.
- app state: lobbyReady (set by Lobby on first settle) + splashDone,
  reset on logout.
- App.svelte: overlay while routeIsLobby && !splashDone; the plain text
  splash now only covers non-lobby deep-links during bootstrap.
- docs: UI_DESIGN + FUNCTIONAL (+ _ru).

ui/src/lib/splash.ts

@@ -0,0 +1,138 @@
+// Pure model for the cold-start loading splash: a small Scrabble crossword that lays the
+// words ЭРУДИТ / ЗАГРУЗКА / ОЖИДАНИЕ out of tiles, letter by letter, until the lobby is
+// ready. Kept free of any DOM or reactive state so it unit-tests in the node environment;
+// components/Splash.svelte wraps it with the reactive reveal set and the timer driver.
+//
+// The tile point values are hardcoded from the Эрудит ruleset (scrabble-solver
+// rules.Erudit). They cannot come from alphabet.valueForLetter() here: the splash renders on
+// a cold start, before the server has sent the variant's alphabet table that lookup reads.
+
+/**
+ * ERUDIT_VALUES is the per-letter point value (вес) for every letter the splash words use,
+ * taken from the Эрудит ruleset. Letters are upper-cased, matching the rest of the UI.
+ */
+export const ERUDIT_VALUES: Record<string, number> = {
+  А: 1,
+  Г: 3,
+  Д: 2,
+  Е: 1,
+  Ж: 5,
+  З: 5,
+  И: 1,
+  К: 2,
+  Н: 1,
+  О: 1,
+  Р: 2,
+  Т: 2,
+  У: 3,
+  Э: 10,
+};
+
+/** SplashWord names the three crossword words, used to group tiles for the reveal sequence. */
+export type SplashWord = 'erudit' | 'zagruzka' | 'ozhidanie';
+
+/** SplashTile is one placed tile on the 6×8 splash grid. */
+export interface SplashTile {
+  /** row is the grid row, 0..7 (ЭРУДИТ sits on row 3). */
+  row: number;
+  /** col is the grid column, 0..5. */
+  col: number;
+  /** letter is the upper-cased Cyrillic glyph shown on the tile. */
+  letter: string;
+  /** value is the tile's Эрудит point value. */
+  value: number;
+  /** word groups the tile by its crossword word. */
+  word: SplashWord;
+  /** key is the stable `"row-col"` identifier used by the renderer's reveal set. */
+  key: string;
+}
+
+/** COLS and ROWS are the splash grid dimensions (6 wide, 8 tall). */
+export const COLS = 6;
+export const ROWS = 8;
+
+function tile(row: number, col: number, letter: string, word: SplashWord): SplashTile {
+  return { row, col, letter, value: ERUDIT_VALUES[letter] ?? 0, word, key: `${row}-${col}` };
+}
+
+// ЭРУДИТ — horizontal on row 3, columns 0..5 (laid left → right).
+const ERUDIT_TILES: SplashTile[] = [...'ЭРУДИТ'].map((l, i) => tile(3, i, l, 'erudit'));
+
+// ЗАГРУЗКА — vertical in column 1, rows 0..7 (laid top → bottom). Its row-3 letter (Р) is the
+// tile already placed by ЭРУДИТ, so it is dropped here ("через имеющиеся буквы").
+const ZAGRUZKA_TILES: SplashTile[] = [...'ЗАГРУЗКА']
+  .map((l, r) => tile(r, 1, l, 'zagruzka'))
+  .filter((t) => t.row !== 3);
+
+// ОЖИДАНИЕ — vertical in column 3, rows 0..7. Its row-3 letter (Д) is shared with ЭРУДИТ and
+// likewise dropped.
+const OZHIDANIE_TILES: SplashTile[] = [...'ОЖИДАНИЕ']
+  .map((l, r) => tile(r, 3, l, 'ozhidanie'))
+  .filter((t) => t.row !== 3);
+
+/** SPLASH_TILES is every unique tile on the grid (ЭРУДИТ owns the two shared crossings). */
+export const SPLASH_TILES: SplashTile[] = [...ERUDIT_TILES, ...ZAGRUZKA_TILES, ...OZHIDANIE_TILES];
+
+/** ERUDIT_KEYS is the set of ЭРУДИТ tile keys — the tiles kept on screen across a clear. */
+export const ERUDIT_KEYS: ReadonlySet<string> = new Set(ERUDIT_TILES.map((t) => t.key));
+
+/** WORD_MS is the time to lay one whole word; PAUSE_MS the gap between words. */
+export const WORD_MS = 1000;
+export const PAUSE_MS = 250;
+
+/** SplashStep is one scheduled action: reveal a tile, check whether the lobby is ready (and
+ *  if so dismiss), or clear every non-ЭРУДИТ tile before the cycle repeats. atMs is relative
+ *  to the start of its segment (prefix or cycle). */
+export interface SplashStep {
+  atMs: number;
+  kind: 'reveal' | 'check' | 'clear';
+  /** key is the tile to reveal, set only on a 'reveal' step. */
+  key?: string;
+}
+
+/** SplashSchedule is the timed plan the renderer drives: the one-time prefix (ЭРУДИТ) and the
+ *  repeating cycle (ЗАГРУЗКА → ОЖИДАНИЕ → clear). The driver plays the prefix once, then loops
+ *  the cycle every cycleMs until the splash is dismissed. */
+export interface SplashSchedule {
+  prefix: SplashStep[];
+  cycle: SplashStep[];
+  prefixMs: number;
+  cycleMs: number;
+}
+
+function revealSteps(tiles: SplashTile[], base: number, perTile: number): SplashStep[] {
+  return tiles.map((t, i) => ({ atMs: Math.round(base + i * perTile), kind: 'reveal', key: t.key }));
+}
+
+/**
+ * splashSchedule builds the reveal plan. Each word is laid over wordMs (so its per-tile
+ * stagger is wordMs divided by the word's tile count), a readiness check lands at every word
+ * boundary, and pauseMs separates the words; after ОЖИДАНИЕ the cycle clears the two vertical
+ * words and restarts (ЭРУДИТ persists). wordMs and pauseMs are overridable for tuning/tests.
+ */
+export function splashSchedule(opts: { wordMs?: number; pauseMs?: number } = {}): SplashSchedule {
+  const wordMs = opts.wordMs ?? WORD_MS;
+  const pauseMs = opts.pauseMs ?? PAUSE_MS;
+
+  // Prefix: ЭРУДИТ left → right, the word completing at wordMs, then a readiness check.
+  const prefix: SplashStep[] = [
+    ...revealSteps(ERUDIT_TILES, 0, wordMs / ERUDIT_TILES.length),
+    { atMs: wordMs, kind: 'check' },
+  ];
+
+  // Cycle: pause, ЗАГРУЗКА, check, pause, ОЖИДАНИЕ, check, pause, clear. The leading pause is
+  // the gap after ЭРУДИТ (and, on repeat, after the previous cycle's clear).
+  const sV = wordMs / ZAGRUZKA_TILES.length;
+  const zBase = pauseMs;
+  const oBase = pauseMs + wordMs + pauseMs;
+  const clearAt = oBase + wordMs + pauseMs;
+  const cycle: SplashStep[] = [
+    ...revealSteps(ZAGRUZKA_TILES, zBase, sV),
+    { atMs: zBase + wordMs, kind: 'check' },
+    ...revealSteps(OZHIDANIE_TILES, oBase, sV),
+    { atMs: oBase + wordMs, kind: 'check' },
+    { atMs: clearAt, kind: 'clear' },
+  ];
+
+  return { prefix, cycle, prefixMs: wordMs, cycleMs: clearAt };
+}