Implement LZSS decompression with optional XOR decryption

- Added `lzss_decompress_simple` function for LZSS decompression in `lzss.rs`.
- Introduced `XorState` struct and `xor_stream` function for XOR decryption in `xor.rs`.
- Updated `mod.rs` to include new LZSS and XOR modules.
- Refactored `parse_library` function in `parse.rs` to utilize the new XOR decryption functionality.
- Cleaned up and organized code in `lib.rs` by removing redundant functions and structures.
- Added tests for new functionality in `tests.rs`.

crates/rsli/src/compress/xor.rs

@@ -0,0 +1,29 @@
+/// XOR cipher state for RsLi format
+pub struct XorState {
+    lo: u8,
+    hi: u8,
+}
+
+impl XorState {
+    /// Create new XOR state from 16-bit key
+    pub fn new(key16: u16) -> Self {
+        Self {
+            lo: (key16 & 0xFF) as u8,
+            hi: ((key16 >> 8) & 0xFF) as u8,
+        }
+    }
+
+    /// Decrypt a single byte and update state
+    pub fn decrypt_byte(&mut self, encrypted: u8) -> u8 {
+        self.lo = self.hi ^ self.lo.wrapping_shl(1);
+        let decrypted = encrypted ^ self.lo;
+        self.hi = self.lo ^ (self.hi >> 1);
+        decrypted
+    }
+}
+
+/// Decrypt entire buffer with XOR stream cipher
+pub fn xor_stream(data: &[u8], key16: u16) -> Vec<u8> {
+    let mut state = XorState::new(key16);
+    data.iter().map(|&b| state.decrypt_byte(b)).collect()
+}