refactor: DDD/CQRS architecture, unified crate layout

- crates: common→application, api→presentation, infrastructure/*→adapters/*
- new crates: api-types, infra-wiring
- domain: errors/, models/, value_objects/, ports/, services/
- application: CQRS use cases (songs/, tabs/) w/ commands, queries, deps
- unified DomainError replaces RepositoryError
- workspace deps, unused dep cleanup
- fix: parse plain-text chord lines (UG drops spans mid-song)
- tests extracted to separate modules (tests/ dirs)

crates/domain/src/lib.rs

@@ -1,13 +1,17 @@
-pub mod note;
-pub mod chord;
-pub mod song;
+pub mod errors;
+pub mod models;
 pub mod ports;
-pub mod transposer;
+pub mod services;
+pub mod value_objects;
 
-pub use note::Note;
-pub use chord::Chord;
-pub use song::{ChordPosition, LyricLine, Section, SectionKind, SongMeta, Song};
-pub use song::{song_preview_chords, StoredSong, SongSummary};
-pub use ports::{FetchError, ParseError, TabFetcherPort, TabParserPort, TabSource};
-pub use ports::{RepositoryError, SongRepositoryPort, SongSearchPort, SortField, SortOrder};
-pub use transposer::{ChordTransposer, TransposeError};
+pub use errors::DomainError;
+pub use models::{
+    ChordPosition, LyricLine, Section, SectionKind, Song, SongMeta, SongSummary, StoredSong,
+    song_preview_chords,
+};
+pub use ports::{
+    FetchError, ParseError, SongRepositoryPort, SongSearchPort, TabFetcherPort, TabParserPort,
+    TabSource,
+};
+pub use services::{ChordTransposer, TransposeError};
+pub use value_objects::{Chord, Note, SortField, SortOrder};