fix(dictionary): stop character dictionary IO from blocking the main process

Generating and importing a large character dictionary froze the whole
app long enough for the compositor to raise its application-not-
responding dialog over the player. Multi-hundred-MB snapshot JSONs and
the merged archive were read, written, and zipped synchronously on the
main process, and the character image / name-candidate caches re-read
every cached snapshot synchronously inside a lookup whenever the
snapshot directory changed.

- snapshot reads/writes are async; writes stream in slices and rename
  into place so a crash or concurrent writer cannot tear a snapshot
- buildDictionaryZip yields between ~8MB slices and CRC32 uses the
  native zlib implementation
- the image and name-candidate lookup caches rebuild in the background
  and serve the previous index while the rebuild runs

Worst main-thread stall over a 1.4GB snapshot set drops from 8s+ to
under 700ms.

src/main/character-dictionary-runtime/zip.test.ts

@@ -42,7 +42,7 @@ function readStoredZipEntries(zipPath: string): Map<string, Buffer> {
   return entries;
 }
 
-test('buildDictionaryZip writes a valid stored zip without fs.writeFileSync', () => {
+test('buildDictionaryZip writes a valid stored zip without fs.writeFileSync', async () => {
   const tempDir = makeTempDir();
   const outputPath = path.join(tempDir, 'dictionary.zip');
   const termEntries: CharacterDictionaryTermEntry[] = [
@@ -62,7 +62,7 @@ test('buildDictionaryZip writes a valid stored zip without fs.writeFileSync', ()
       );
     }) as typeof Buffer.concat;
 
-    const result = buildDictionaryZip(
+    const result = await buildDictionaryZip(
       outputPath,
       'Dictionary Title',
       'Dictionary Description',
@@ -106,11 +106,11 @@ test('buildDictionaryZip writes a valid stored zip without fs.writeFileSync', ()
   }
 });
 
-test('readDictionaryZipRevision reads the built revision and rejects foreign archives', () => {
+test('readDictionaryZipRevision reads the built revision and rejects foreign archives', async () => {
   const dir = makeTempDir();
   try {
     const zipPath = path.join(dir, 'merged.zip');
-    buildDictionaryZip(
+    await buildDictionaryZip(
       zipPath,
       'SubMiner Character Dictionary',
       'Character names',