fix: replace panic with error return in execute(), fix Pull/Push/Reboot/Root targeting

- execute() no longer panics when adb is not found; returns ErrNotInstalled
- Use sync.Once for lazy adb path lookup instead of init()
- Fix Pull() checking src (device path) instead of dest (local path)
- Add ErrDestExists error for Pull destination conflicts
- Add -s serial flag to Push, Pull, Reboot, Root for multi-device support
- Root() now parses stdout to determine actual success
- Fix Shell() redundant variable redeclaration
- Fix 'inconsostency' typo in KillServer doc
- Add doc comments to all error variables
- Update Go 1.26.0 -> 1.26.1
- Add tests: ConnString, ShortenSleep, GetLength, JSON roundtrip,
  SequenceImporter.ToInput, insertSleeps, parseDevices edge cases,
  parseScreenResolution edge cases, filterErr

shell_test.go

@@ -18,7 +18,9 @@ func Test_parseScreenResolution(t *testing.T) {
 		{name: "Pixel 4XL", args: args{in: "Physical size: 1440x3040"}, wantRes: Resolution{Width: 1440, Height: 3040}, wantErr: false},
 		{name: "Pixel XL", args: args{in: "Physical size: 1440x2560"}, wantRes: Resolution{Width: 1440, Height: 2560}, wantErr: false},
 		{name: "garbage", args: args{in: "asdfhjkla"}, wantRes: Resolution{Width: -1, Height: -1}, wantErr: true},
-		// TODO: Add test cases.
+		{name: "empty string", args: args{in: ""}, wantRes: Resolution{Width: -1, Height: -1}, wantErr: true},
+		{name: "Samsung S21", args: args{in: "Physical size: 1080x2400"}, wantRes: Resolution{Width: 1080, Height: 2400}, wantErr: false},
+		{name: "override size", args: args{in: "Physical size: 1440x3120\nOverride size: 1080x2340"}, wantRes: Resolution{Width: 1440, Height: 3120}, wantErr: false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {