Rename Pi tarball to stegasoo-rpi-runtime-env-arm64.tar.zst

More descriptive name for the pre-built pyenv + venv bundle.
Updated all scripts and docs to use new filename.
Also bumped PREBUILT_URL to v4.1.5.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

rpi/kickoff-pi-test.sh

@@ -154,7 +154,7 @@ echo ""
 echo -e "${GREEN}[6/8]${NC} Copying pre-built tarball to Pi..."
 echo ""
 
-TARBALL="$SCRIPT_DIR/stegasoo-pi-arm64.tar.zst"
+TARBALL="$SCRIPT_DIR/stegasoo-rpi-runtime-env-arm64.tar.zst"
 if [[ -f "$TARBALL" ]]; then
     scp_to_pi "$TARBALL" "/opt/stegasoo/rpi/"
     echo -e "  ${GREEN}✓${NC} Tarball copied"