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/adapters/ug-parser/src/tests/fetcher.rs

@@ -0,0 +1,19 @@
+use super::*;
+use domain::TabSource;
+use std::path::PathBuf;
+
+#[tokio::test]
+async fn fetch_local_file() {
+    let fetcher = UgTabFetcher::new();
+    let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .parent()
+        .unwrap()
+        .parent()
+        .unwrap()
+        .parent()
+        .unwrap()
+        .join("samples/drop_in_the_ocean.html");
+    let html = fetcher.fetch(TabSource::File(path)).await.unwrap();
+    assert!(html.contains("[Chorus]"));
+    assert!(html.contains("data-name=\"Em\""));
+}