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.

tools/zipfetch.py

@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""Estrae singoli file da uno ZIP remoto via HTTP Range (senza scaricare 1,28 GB).
+
+Il tpz dei template Godot e' uno zip: serve solo templates/linux_release.arm64.
+Legge EOCD + central directory con Range, poi scarica solo il membro richiesto.
+"""
+import binascii
+import struct
+import sys
+import urllib.request
+import zlib
+
+URL = ("https://github.com/godotengine/godot-builds/releases/download/"
+       "4.7.1-stable/Godot_v4.7.1-stable_export_templates.tpz")
+
+
+def get_range(url, start, end=None):
+    if end is None:
+        rng = f"bytes={start}-"
+    else:
+        rng = f"bytes={start}-{end}"
+    req = urllib.request.Request(url, headers={"Range": rng})
+    with urllib.request.urlopen(req, timeout=90) as r:
+        return r.read(), r.status
+
+
+def main(wanted, dest_dir):
+    # 1. dimensione totale
+    req = urllib.request.Request(URL, method="HEAD")
+    with urllib.request.urlopen(req, timeout=60) as r:
+        total = int(r.headers["Content-Length"])
+    print(f"zip remoto: {total} byte")
+
+    # 2. EOCD negli ultimi 64 KiB
+    tail_len = min(65536, total)
+    tail, _ = get_range(URL, total - tail_len)
+    pos = tail.rfind(b"PK\x05\x06")
+    if pos < 0:
+        sys.exit("EOCD non trovato (zip64?)")
+    (_, _, _, _, n_entries, cd_size, cd_off, _) = struct.unpack(
+        "<IHHHHIIH", tail[pos:pos + 22])
+    print(f"voci: {n_entries}, central directory: {cd_off}+{cd_size}")
+
+    # 3. central directory
+    cd, _ = get_range(URL, cd_off, cd_off + cd_size - 1)
+
+    entries = {}
+    i = 0
+    while i + 46 <= len(cd) and cd[i:i + 4] == b"PK\x01\x02":
+        (method, csize, usize, name_len, extra_len, comment_len,
+         local_off) = (struct.unpack("<H", cd[i + 10:i + 12])[0],
+                       struct.unpack("<I", cd[i + 20:i + 24])[0],
+                       struct.unpack("<I", cd[i + 24:i + 28])[0],
+                       struct.unpack("<H", cd[i + 28:i + 30])[0],
+                       struct.unpack("<H", cd[i + 30:i + 32])[0],
+                       struct.unpack("<H", cd[i + 32:i + 34])[0],
+                       struct.unpack("<I", cd[i + 42:i + 46])[0])
+        name = cd[i + 46:i + 46 + name_len].decode("utf-8", "replace")
+        entries[name] = (method, csize, usize, local_off)
+        i += 46 + name_len + extra_len + comment_len
+
+    print(f"membri indicizzati: {len(entries)}")
+    if wanted not in entries:
+        candidati = [n for n in entries if "arm64" in n]
+        sys.exit(f"'{wanted}' non presente. Candidati arm64: {candidati}")
+
+    method, csize, usize, local_off = entries[wanted]
+    print(f"{wanted}: metodo={method} compresso={csize} originale={usize}")
+
+    # 4. header locale per sapere dove iniziano davvero i dati
+    lh, _ = get_range(URL, local_off, local_off + 29)
+    if lh[:4] != b"PK\x03\x04":
+        sys.exit("header locale inatteso")
+    name_len = struct.unpack("<H", lh[26:28])[0]
+    extra_len = struct.unpack("<H", lh[28:30])[0]
+    data_start = local_off + 30 + name_len + extra_len
+
+    # 5. dati compressi del solo membro
+    print(f"scarico {csize} byte da offset {data_start} ...")
+    data, status = get_range(URL, data_start, data_start + csize - 1)
+    print(f"ricevuti {len(data)} byte (http {status})")
+
+    if method == 8:
+        raw = zlib.decompress(data, -15)
+    elif method == 0:
+        raw = data
+    else:
+        sys.exit(f"metodo di compressione non gestito: {method}")
+
+    print(f"decompresso: {len(raw)} byte (atteso {usize})")
+    if len(raw) != usize:
+        sys.exit("dimensione non corrispondente")
+    out = f"{dest_dir}/{wanted.split('/')[-1]}"
+    with open(out, "wb") as fh:
+        fh.write(raw)
+    print(f"scritto {out}")
+
+
+if __name__ == "__main__":
+    target = sys.argv[1] if len(sys.argv) > 1 else "templates/linux_release.arm64"
+    dest = sys.argv[2] if len(sys.argv) > 2 else "/tmp/godot-dl"
+    main(target, dest)