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/crates/psysonic-core/src/lib.rs

@@ -0,0 +1,8 @@
+//! `psysonic-core` — workspace-internal shared primitives.
+//!
+//! Hosts the runtime logging facade (with `app_eprintln!` / `app_deprintln!`
+//! macros) and the cross-crate port traits used to break dependency cycles
+//! between `psysonic-audio`, `psysonic-analysis`, and other domain crates.
+
+pub mod logging;
+pub mod ports;