Complete API standardization and cleanup tracking docs

API Improvements:
- Add OriginalReleaseDate property to Tag base class
- Add Lyrics, UniqueFileIdentifiers to Tag base class
- Standardize error messages: "Invalid {format} file: {reason}"
- Add IFileSystem storage to WavFile and AiffFile for SaveToFileAsync
- Add IsValidFormat static methods to all file types
- Add MP4 atom mapping for ORIGINALDATE

Documentation Cleanup:
- Delete API_TODO.md (all 18 items completed)
- Delete API_STANDARDIZATION.md (all 4 items completed)
- Delete MILESTONES.md (historical, no longer needed)
- Update ROADMAP.md with verified completion status

Test Improvements:
- Add IsValidFormatTests for all 14 file formats
- Add extended metadata tests for Lyrics/UniqueFileId
- Add public API snapshot for .NET 8.0

Author: decriptor

API_STANDARDIZATION.md

@@ -548,7 +548,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - DateTagged (TDTG / DATETAGGED) - ISO 8601 tagging timestamp (ID3v2.4 only)
 - Description (TXXX:DESCRIPTION / DESCRIPTION) - Content synopsis
 - AmazonId (TXXX:ASIN / ASIN) - Amazon Standard Identification Number
-- MusicIpId (TXXX:MusicIP PUID / MUSICIP_PUID) - Obsolete, deprecated in favor of AcoustID
+- ~~MusicIpId~~ - Removed (MusicIP service discontinued, use AcoustID instead)
 - Extended MusicBrainz identifiers (ID3v2 and Vorbis Comments):
   - WorkId - Musical work/composition identifier
   - DiscId - CD table of contents hash