fix: Steam Deck/Ubuntu crash with jemalloc allocator

Root cause: glibc 2.41 has stricter heap validation that catches a
pre-existing race condition triggered by binary patching.

Changes:
- Add jemalloc auto-detection and usage on Linux
- Add auto-install via pkexec (graphical sudo prompt)
- Clean up clientPatcher.js (remove debug env vars)
- Add null-padding fix for shorter domain replacements
- Document investigation and solution

The launcher now:
1. Auto-detects jemalloc if installed
2. Offers to auto-install if missing (password prompt)
3. Falls back to MALLOC_CHECK_=0 if jemalloc unavailable

Install manually: sudo pacman -S jemalloc (Arch/Steam Deck)
                  sudo apt install libjemalloc2 (Debian/Ubuntu)

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

docs/STEAMDECK_DEBUG_COMMANDS.md

@@ -0,0 +1,95 @@
+# Steam Deck / Linux Crash Fix
+
+## SOLUTION: Use jemalloc ✓
+
+The crash is caused by glibc 2.41's stricter heap validation. Using jemalloc as the memory allocator fixes the issue.
+
+### Install jemalloc
+
+```bash
+# Steam Deck / Arch Linux
+sudo pacman -S jemalloc
+
+# Ubuntu / Debian
+sudo apt install libjemalloc2
+
+# Fedora / RHEL
+sudo dnf install jemalloc
+```
+
+### Launcher Auto-Detection
+
+The launcher automatically uses jemalloc when installed. No manual configuration needed.
+
+To disable (for testing):
+```bash
+HYTALE_NO_JEMALLOC=1 npm start
+```
+
+### Manual Launch with jemalloc
+
+```bash
+cd ~/.hytalef2p/release/package/game/latest
+
+LD_PRELOAD=/usr/lib/libjemalloc.so.2 ./Client/HytaleClient --app-dir /home/deck/.hytalef2p/release/package/game/latest --java-exec /home/deck/.hytalef2p/release/package/jre/latest/bin/java --auth-mode authenticated --uuid YOUR_UUID --name Player --identity-token YOUR_TOKEN --session-token YOUR_TOKEN --user-dir /home/deck/.hytalesaves
+```
+
+---
+
+## Debug Commands (for troubleshooting)
+
+### Base Command
+```bash
+cd ~/.hytalef2p/release/package/game/latest
+```
+
+### GDB Stack Trace (for crash analysis)
+```bash
+gdb -ex "run --app-dir ..." ./Client/HytaleClient
+
+# After crash:
+bt
+bt full
+info registers
+quit
+```
+
+### Test glibc tunables (alternative fixes that didn't work reliably)
+
+**Disable tcache:**
+```bash
+GLIBC_TUNABLES=glibc.malloc.tcache_count=0 ./Client/HytaleClient ...
+```
+
+**Disable heap validation:**
+```bash
+MALLOC_CHECK_=0 ./Client/HytaleClient ...
+```
+
+### Binary Validation
+```bash
+file ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+ldd ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+```
+
+### Hex Dump Commands
+```bash
+# Search for hytale.com UTF-16LE
+xxd ~/.hytalef2p/release/package/game/latest/Client/HytaleClient.original | grep "6800 7900 7400 6100 6c00 6500 2e00 6300 6f00 6d00"
+```
+
+---
+
+## Test Different Patch Modes
+
+```bash
+# Restore original
+cp ~/.hytalef2p/release/package/game/latest/Client/HytaleClient.original ~/.hytalef2p/release/package/game/latest/Client/HytaleClient
+rm ~/.hytalef2p/release/package/game/latest/Client/HytaleClient.patched_custom
+
+# Test UTF-16LE mode
+HYTALE_PATCH_MODE=utf16le HYTALE_AUTH_DOMAIN=sanasol.ws npm start
+
+# Test length-prefixed mode (default)
+HYTALE_AUTH_DOMAIN=sanasol.ws npm start
+```