feat(fls): ZIP layer of the .fls codec

Reader and writer ported from filius-ipad with the same path policy,
limits and error codes; DEFLATE via fflate, cut at the declared size
like Apple Compression. 54 tests: the Swift archive rejection tests,
DEFLATE edge cases, and all 23 corpus files cross-checked against
fflate.unzipSync and round-tripped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/fls/test/corpus.test.ts

@@ -0,0 +1,46 @@
+// The 23 real .fls files of the corpus (corpus/README.md): every one opens,
+// the entries match what an independent ZIP reader extracts, and writing
+// them back and reading again changes nothing.
+
+import { readdirSync, readFileSync, statSync } from 'node:fs';
+import { dirname, join, relative } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { unzipSync } from 'fflate';
+import { describe, expect, it } from 'vitest';
+import { CONFIGURATION_ENTRY_PATH, readFlsArchive, writeFlsArchive } from '../src/index.js';
+
+const corpusRoot = join(dirname(fileURLToPath(import.meta.url)), '../../../corpus');
+
+function flsFiles(directory: string): string[] {
+  return readdirSync(directory)
+    .flatMap((name) => {
+      const path = join(directory, name);
+      return statSync(path).isDirectory() ? flsFiles(path) : name.endsWith('.fls') ? [path] : [];
+    })
+    .sort();
+}
+
+const files = flsFiles(corpusRoot);
+
+describe('corpus archives', () => {
+  it('has all 23 files', () => {
+    expect(files).toHaveLength(23);
+  });
+
+  for (const file of files) {
+    it(relative(corpusRoot, file), () => {
+      const bytes = new Uint8Array(readFileSync(file));
+      const contents = readFlsArchive(bytes);
+
+      const independent = unzipSync(bytes);
+      const independentFiles = Object.keys(independent).filter((path) => !path.endsWith('/')).sort();
+      expect(contents.entryPaths).toEqual(independentFiles);
+      expect(contents.configurationXml).toEqual(independent[CONFIGURATION_ENTRY_PATH]);
+      for (const [path, data] of contents.supplementalEntries) expect(data, path).toEqual(independent[path]);
+
+      const rewritten = readFlsArchive(writeFlsArchive(contents.configurationXml, contents.supplementalEntries));
+      expect(rewritten.configurationXml).toEqual(contents.configurationXml);
+      expect(rewritten.supplementalEntries).toEqual(contents.supplementalEntries);
+    });
+  }
+});