refactor: introduce cargo workspace + psysonic-core crate (M1/7)

First milestone of the workspace crate split. Sets up the workspace
skeleton and extracts shared logging + cross-crate port traits into a
new psysonic-core crate.

  src-tauri/Cargo.toml            now defines [workspace]; top package
                                  becomes the workspace root.
  crates/psysonic-core/           new lib crate, no Tauri-handler code:
    src/logging.rs                full logging facade (was src/logging.rs)
    src/ports.rs                  PlaybackQuery + AnalysisOrchestrator
                                  trait declarations (no impls yet)

The top crate re-exports `psysonic_core::logging` and the
`app_eprintln!` / `app_deprintln!` macros so every existing
`crate::logging::*` and `crate::app_eprintln!` callsite keeps working
unchanged.

Port traits intentionally take `tauri::AppHandle` — psysonic-core is a
workspace-internal crate, so depending on Tauri here is a feature, not
a leak. Implementers will register themselves as
`Arc<dyn PlaybackQuery>` / `Arc<dyn AnalysisOrchestrator>` Tauri State
in M2/M3.

Behaviour preserving. Cargo check + clippy --workspace clean.

src-tauri/src/lib.rs

@@ -6,8 +6,10 @@ mod analysis_cache;
 mod analysis_runtime;
 pub mod cli;
 mod discord;
-pub(crate) mod logging;
 mod lib_commands;
+
+pub use psysonic_core::logging;
+pub use psysonic_core::{app_eprintln, app_deprintln};
 #[cfg(target_os = "windows")]
 mod taskbar_win;
 mod tray_runtime;