fix: Steam Deck/Ubuntu crash - use system libzstd.so

The bundled libzstd.so is incompatible with glibc 2.41's stricter heap
validation, causing "free(): invalid pointer" crashes.

Solution: Automatically replace bundled libzstd.so with system version
on Linux. The launcher detects and symlinks to /usr/lib/libzstd.so.1.

- Auto-detect system libzstd at common paths (Arch, Debian, Fedora)
- Backup bundled version as libzstd.so.bundled
- Create symlink to system version
- Add HYTALE_NO_LIBZSTD_FIX=1 to disable if needed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/STEAMDECK_DEBUG_COMMANDS.md

@@ -0,0 +1,65 @@
+# Steam Deck / Linux Crash Fix
+
+## SOLUTION: Use system libzstd
+
+The crash is caused by the bundled `libzstd.so` being incompatible with glibc 2.41's stricter heap validation.
+
+### Automatic Fix
+
+The launcher automatically replaces `libzstd.so` with the system version. No manual action needed.
+
+### Manual Fix
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+
+# Backup and replace
+mv libzstd.so libzstd.so.bundled
+ln -s /usr/lib/libzstd.so.1 libzstd.so
+```
+
+### Restore Original
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+rm libzstd.so
+mv libzstd.so.bundled libzstd.so
+```
+
+---
+
+## Debug Commands (for troubleshooting)
+
+### Check libzstd Status
+
+```bash
+# Check if symlinked
+ls -la ~/.hytalef2p/release/package/game/latest/Client/libzstd.so
+
+# Find system libzstd
+find /usr/lib -name "libzstd.so*"
+```
+
+### Binary Validation
+
+```bash
+file ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+ldd ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+```
+
+### Restore Client Binary
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest/Client
+cp HytaleClient.original HytaleClient
+rm -f HytaleClient.patched_custom
+```
+
+---
+
+## Environment Variables
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `HYTALE_AUTH_DOMAIN` | Custom auth domain | `auth.sanasol.ws` |
+| `HYTALE_NO_LIBZSTD_FIX` | Disable libzstd replacement | `1` |