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/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "@mdg-filius/fls",
+  "version": "0.0.0",
+  "private": true,
+  "license": "GPL-3.0-only",
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "dependencies": {
+    "fflate": "^0.8.2"
+  }
+}