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/value_objects/mod.rs

@@ -0,0 +1,7 @@
+mod chord;
+mod note;
+mod sorting;
+
+pub use chord::*;
+pub use note::*;
+pub use sorting::*;