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>

Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "appledouble"
+version = "0.1.0"
+edition = "2021"
+description = "AppleSingle/AppleDouble container codec: exhaustive parse, lossless build"
+license = "MIT"
+
+[dependencies]
+binrw = "0.15"
+thiserror = "2"