1.1 MacAddress value type

Permissive parser (colon, hyphen, unseparated; case-insensitive; surrounding
whitespace) with canonical lowercase no-separator ToString. Implements
IParsable/ISpanParsable for ASP.NET route binding and ships a System.Text.Json
converter that normalizes on read.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

IMPLEMENTATION.md

@@ -33,7 +33,7 @@ When working through this file, mark increments complete by changing `[ ]` to `[
 
 Each type lives in `src/FrameProcessor/Domain/`. Tests in `tests/FrameProcessor.Tests/`.
 
-### [ ] 1.1 `MacAddress`
+### [x] 1.1 `MacAddress`
 - `record struct` storing 6 bytes.
 - `static bool TryParse(string, out MacAddress)` accepting `AA:BB:CC:DD:EE:FF`, `aa-bb-cc-dd-ee-ff`, `AABBCCDDEEFF`, surrounding whitespace.
 - `ToString()` → canonical lowercase no-separator (`aabbccddeeff`).