Add Linux ARM64 preset, Raspberry Pi 5 docs and remote zip fetcher

- export_presets.cfg: new "Linux ARM64" preset (arch arm64, embedded PCK,
  ETC2/ASTC textures, S3TC/BPTC off since the Pi 5 V3D driver lacks it).
- tools/zipfetch.py: pull a single member out of the remote export-templates
  tpz with HTTP Range requests instead of downloading the whole 1.28 GB.
- docs/BUILD-RASPBERRY-PI.md: reproducible procedure, Pi 5 renderer
  expectations (GLES3 fallback, Forward+ downgraded to Mobile) and the
  pitfalls verified on the field.

Verified: arm64 export produced (74.094.536 B, PCK embedded at the end of the
official template); the x86_64 export from the same tree runs without missing
resources.

export_presets.cfg

@@ -54,3 +54,31 @@ binary_format/architecture="x86_64"
 texture_format/s3tc_bptc=true
 texture_format/etc2_astc=false
 ssh_remote_deploy/enabled=false
+
+[preset.2]
+
+name="Linux ARM64"
+platform="Linux"
+runnable=true
+advanced_options=true
+dedicated_server=false
+custom_features=""
+export_filter="all_resources"
+include_filter=""
+exclude_filter="addons/*,tools/*,docs/*,build/*,assets/**/*.mdl,assets/half_life_manifest.json"
+export_path="build/linux-arm64/work_death_balance.arm64"
+patches=PackedStringArray()
+encrypt_pck=false
+encrypt_directory=false
+script_export_mode=2
+
+[preset.2.options]
+
+custom_template/release=""
+custom_template/debug=""
+debug/export_console_wrapper=0
+binary_format/embed_pck=true
+binary_format/architecture="arm64"
+texture_format/s3tc_bptc=false
+texture_format/etc2_astc=true
+ssh_remote_deploy/enabled=false