packaging/windows: fix BTRFS+QEMU boot-loop, document what's confirmed

Real issue hit on a real run: dockur/windows warns about BTRFS storage
but it's not idle -- on this host it boot-looped Windows Setup for
hours (same log lines repeating forever, disk barely growing), a known
bad combination for QEMU disk images on a copy-on-write filesystem.
build_windows.sh now disables COW on storage/ itself (chattr +C,
harmless no-op on non-btrfs or an already-populated dir from a prior
run). README updated to reflect both real fixes now confirmed needed
on this host (this one, plus the earlier SELinux :Z mount fix) instead
of the original "written but never run" status.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/windows/build_windows.sh

@@ -30,6 +30,17 @@ fi
 command -v docker >/dev/null 2>&1 || { echo "docker not found." >&2; exit 1; }
 
 mkdir -p storage oem shared/src shared/dist "$OUT_DIR"
+# dockur/windows itself warns about this ("you are using the BTRFS
+# filesystem for /storage, this might introduce issues with Windows
+# Setup!") and it's not idle: confirmed on this host as a genuine
+# multi-hour Windows Setup boot-loop (repeating the same boot-manager
+# log lines forever, disk barely growing) -- QEMU disk images on a
+# copy-on-write filesystem are a known bad combination. +C only takes
+# effect for files created AFTER it's set on an empty directory, so
+# this only helps on a fresh/emptied storage/; it's a no-op (harmless,
+# chattr just errors quietly) on a non-btrfs filesystem or an
+# already-populated storage/ from a previous run.
+chattr +C storage 2>/dev/null || true
 
 echo "==> starting the Windows build VM (docker compose up -d)"
 docker compose up -d