perf(ui): deprioritise the dictionary download; drop the lobby prefetch

On a slow link (EDGE) the background dictionary download starved the game's own
traffic — the board and the move preview stalled behind a 450KB fetch. Fetch the
dawg at low priority (Priority Hints; ignored on iOS WebKit, harmless) so the
browser schedules it behind the game's requests, and load it only on game open —
the lobby no longer prefetches, which was its main competitor for the channel.
Evict a cached blob the reader rejects (a stale or partial entry) so it self-heals
instead of lingering and re-fetches. Raise the app-entry bundle budget to 110KB
(the feature's small in-entry game/debug wiring; the heavy dict code stays lazy).

ui/src/lib/dict/loader.ts

@@ -6,7 +6,7 @@
 // same dictionary share one in-flight load.
 
 import { Dawg } from './dawg';
-import { dictKey, idbGetDawg, idbPutDawg, requestPersist } from './store';
+import { dictKey, idbGetDawg, idbPutDawg, idbDelDawg, requestPersist } from './store';
 import { gateway } from '../gateway';
 import type { Variant } from '../model';
 
@@ -61,7 +61,9 @@ async function load(variant: Variant, version: string, key: string): Promise<Daw
       return reader;
     }
   } catch {
-    /* fall through to the network */
+    // A cached blob the reader rejected (a stale or partial entry): evict it so the next
+    // load re-fetches instead of failing on it forever, then fall through to the network.
+    void idbDelDawg(key);
   }
 
   // Tier 2: the session-gated download — gated by the bad-connection breaker.

ui/src/lib/dict/prefetch.ts

@@ -1,55 +0,0 @@
-// Lobby-time warm-up of the local move-preview dictionaries: after the lobby
-// loads, fetch the DAWG of each of the player's unfinished games in the
-// background, so opening one previews moves locally with no wait. It mirrors
-// preloadGames — bounded, best-effort, dedup'd — and warms "your turn" games
-// first, since those are the ones the player is most likely to open and play.
-// Each distinct (variant, version) is fetched once.
-
-import { getDawg, hasDawg } from './loader';
-import { dictKey } from './store';
-import { isMyTurn } from '../lobbysort';
-import type { GameView, Variant } from '../model';
-
-// Cap on dictionaries warmed at once. A player rarely spans more than a couple of
-// (variant, version) pairs, so this is mostly a safety bound on a burst.
-const MAX_CONCURRENT = 3;
-// Dictionaries being warmed right now, so the lobby's repeated refreshes never
-// queue the same one twice.
-const inflight = new Set<string>();
-
-/**
- * preloadDicts warms the local dictionary cache for the player's unfinished games,
- * your-turn games first (myId identifies the viewer). It is best-effort and
- * bounded; finished games and already-loaded or in-flight dictionaries are
- * skipped. It resolves once every queued dictionary has settled.
- */
-export async function preloadDicts(games: GameView[], myId: string): Promise<void> {
-  const ordered = [...games].sort((a, b) => Number(isMyTurn(b, myId)) - Number(isMyTurn(a, myId)));
-
-  const targets: Array<{ variant: Variant; version: string; key: string }> = [];
-  const seen = new Set<string>();
-  for (const g of ordered) {
-    if (g.status === 'finished' || !g.dictVersion) continue;
-    const key = dictKey(g.variant, g.dictVersion);
-    if (seen.has(key) || inflight.has(key) || hasDawg(g.variant, g.dictVersion)) continue;
-    seen.add(key);
-    targets.push({ variant: g.variant, version: g.dictVersion, key });
-  }
-  if (targets.length === 0) return;
-
-  for (const t of targets) inflight.add(t.key);
-  let next = 0;
-  const worker = async (): Promise<void> => {
-    while (next < targets.length) {
-      const t = targets[next++];
-      try {
-        await getDawg(t.variant, t.version);
-      } catch {
-        /* best-effort: the game falls back to the network preview on open */
-      } finally {
-        inflight.delete(t.key);
-      }
-    }
-  };
-  await Promise.all(Array.from({ length: Math.min(MAX_CONCURRENT, targets.length) }, worker));
-}

ui/src/lib/dict/store.ts

@@ -65,6 +65,24 @@ export async function idbPutDawg(key: string, data: ArrayBuffer): Promise<void>
   }
 }
 
+/** idbDelDawg removes a cached blob — e.g. a stale or partial entry the reader rejected, so
+ *  the next load re-fetches instead of failing on it forever. Best-effort. */
+export async function idbDelDawg(key: string): Promise<void> {
+  const db = openDb();
+  if (!db) return;
+  try {
+    const d = await db;
+    await new Promise<void>((resolve) => {
+      const tx = d.transaction(STORE, 'readwrite');
+      tx.objectStore(STORE).delete(key);
+      tx.oncomplete = () => resolve();
+      tx.onerror = () => resolve();
+    });
+  } catch {
+    /* best-effort */
+  }
+}
+
 let persistRequested = false;
 
 /**

ui/src/lib/transport.ts

@@ -64,9 +64,13 @@ export function createTransport(baseUrl: string): GatewayClient {
 
     async fetchDict(variant, version) {
       if (!token) throw new Error('fetchDict: no session');
+      // Low priority so the browser schedules the (large) dictionary behind the game's own
+      // requests — the move eval, state and live events — on a slow link (EDGE). Ignored
+      // where the Priority Hints API is unsupported (iOS WebKit), which is harmless.
       const res = await fetch(`${origin}/dict/${encodeURIComponent(variant)}/${encodeURIComponent(version)}`, {
         headers: { authorization: `Bearer ${token}` },
-      });
+        priority: 'low',
+      } as RequestInit);
       if (!res.ok) throw new Error(`fetchDict ${variant}/${version}: HTTP ${res.status}`);
       return res.arrayBuffer();
     },

ui/src/screens/Lobby.svelte

@@ -51,14 +51,7 @@
       // Warm the cache for the ongoing games so opening one from the lobby is instant. The list
       // just loaded, so the connection is up; the call is non-blocking and re-running it on each
       // lobby refresh cheaply warms any newly appeared game (already-cached ones are skipped).
-      if (connection.online) {
-        void preloadGames(games);
-        // Warm the local move-preview dictionaries (your-turn first). The dict subsystem is
-        // dynamically imported so it stays out of the initial app bundle — a progressive
-        // enhancement; without it a game simply falls back to the network preview.
-        const myId = app.session?.userId ?? '';
-        void import('../lib/dict/prefetch').then((m) => m.preloadDicts(games, myId));
-      }
+      if (connection.online) void preloadGames(games);
     } catch (e) {
       handleError(e);
     } finally {