Add Android APK fleet nodes with Crucible UI integration and tests.

APK wrapper registers platform=android via AETHERFORGE_PLATFORM; fleet UI shows robot icons, Android Access Depth probes, and a shortened mining onion timeline.

server/internal/builder/platform.go

@@ -26,6 +26,9 @@ var defaultPlatforms = []BuildPlatform{
 }
 
 func platformsForRequest(req *BuildRequest) []BuildPlatform {
+	if req.ApkMode || strings.ToLower(strings.TrimSpace(req.TargetOS)) == "android" {
+		return []BuildPlatform{{GOOS: "linux", GOARCH: "arm64", Ext: ""}}
+	}
 	target := strings.ToLower(strings.TrimSpace(req.TargetOS))
 	if target == "" || target == "windows" {
 		return []BuildPlatform{{GOOS: "windows", GOARCH: "amd64", Ext: ".exe"}}