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/Cargo.lock

@@ -3608,6 +3608,7 @@ dependencies = [
  "libc",
  "lofty",
  "md5",
+ "psysonic-core",
  "reqwest",
  "ringbuf",
  "rodio",
@@ -3637,6 +3638,15 @@ dependencies = [
  "zbus 5.15.0",
 ]
 
+[[package]]
+name = "psysonic-core"
+version = "1.46.0-dev"
+dependencies = [
+ "libc",
+ "serde",
+ "tauri",
+]
+
 [[package]]
 name = "pxfm"
 version = "0.1.29"