appledouble: initial commit, extracted from ad-decoder

The codec module (src/appledouble.rs) lifted verbatim from ad-decoder's
working tree at extraction time — including its uncommitted entry-ID
constant additions — with only the crate-level docs rewritten. 17
in-module tests carried over and green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -0,0 +1,17 @@
+# appledouble
+
+AppleSingle/AppleDouble container codec: exhaustive parse, lossless build.
+
+Extracted from [ad-decoder] as a shared crate. Entries decode into a
+`BTreeMap<u32, Vec<u8>>` with typed views over Real Name (3), Comment (4),
+File Dates (8), Finder Info (9), and Macintosh File Info (10) — everything
+else is preserved as raw bytes, so parse → build round trips are lossless
+even for entry types this crate has no opinion about. The macOS `ATTR`
+extension (arbitrary xattrs packed after Finder Info) is parsed and rebuilt.
+
+Consumers:
+- **ad-decoder** — assertive (non-obliterative) sidecar folding onto APFS
+- **adx** — AS/AD inspection and editing
+- **fsinspect** — FUSE-side AppleDouble synthesis for Linux EA recovery
+
+[ad-decoder]: https://code.movq.us/movq/ad-decoder