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/application/src/tabs/commands.rs

@@ -0,0 +1,4 @@
+pub struct ParseTabCommand {
+    pub source: Option<String>,
+    pub html: Option<String>,
+}

crates/application/src/tabs/deps.rs

@@ -0,0 +1,7 @@
+use domain::ports::{TabFetcherPort, TabParserPort};
+use std::sync::Arc;
+
+pub struct ParseTabDeps {
+    pub fetcher: Arc<dyn TabFetcherPort>,
+    pub parser: Arc<dyn TabParserPort>,
+}

crates/application/src/tabs/mod.rs

@@ -0,0 +1,3 @@
+pub mod commands;
+pub mod deps;
+pub mod parse_tab;

crates/application/src/tabs/parse_tab.rs

@@ -0,0 +1,33 @@
+use std::path::PathBuf;
+
+use domain::errors::DomainError;
+use domain::models::Song;
+use domain::ports::TabSource;
+
+use super::commands::ParseTabCommand;
+use super::deps::ParseTabDeps;
+
+pub async fn execute(deps: &ParseTabDeps, cmd: ParseTabCommand) -> Result<Song, DomainError> {
+    let html = if let Some(raw_html) = cmd.html {
+        Ok(raw_html)
+    } else if let Some(source) = cmd.source {
+        let tab_source = if source.starts_with("file://") {
+            let path = source.trim_start_matches("file://");
+            TabSource::File(PathBuf::from(path))
+        } else {
+            TabSource::Url(source)
+        };
+        deps.fetcher
+            .fetch(tab_source)
+            .await
+            .map_err(|e| DomainError::InfrastructureError(e.to_string()))
+    } else {
+        Err(DomainError::ValidationError(
+            "Provide either 'source' or 'html'".into(),
+        ))
+    }?;
+
+    deps.parser
+        .parse(&html)
+        .map_err(|e| DomainError::InfrastructureError(e.to_string()))
+}