Initial commit: xenia-rs workspace for Xbox 360 RE

Rust reimplementation of the xenia Xbox 360 emulator targeting reverse-
engineering and preservation, initially scoped to Project Sylpheed.

Includes:
- XEX2 loader (LZX decompression, AES decryption, PE parsing)
- XISO / XGD2 disc image VFS
- PPC interpreter with 200+ opcodes and VMX128 decoding
- Static analyzer: functions, cross-references, labels, asm + SQLite output
- HLE kernel covering the xboxkrnl/xam subset used by Sylpheed init
- Debugger with in-memory and SQLite-backed execution tracing
- `xenia-rs` CLI with extract/dis/exec commands that produce cumulative,
  superset SQLite databases and opt-in instruction/import/branch traces

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/xenia-apu/src/lib.rs

@@ -0,0 +1,16 @@
+/// Audio processing unit stub. Logging only for now.
+pub struct AudioSystem {
+    pub enabled: bool,
+}
+
+impl AudioSystem {
+    pub fn new() -> Self {
+        Self { enabled: false }
+    }
+}
+
+impl Default for AudioSystem {
+    fn default() -> Self {
+        Self::new()
+    }
+}