feat: --clear-data flag + typed attribute selectors (#48)

* feat(test): add --clear-data flag to clear app data on launch

* test+docs: cover --clear-data flag in CLI parser test and reference

* feat(spec): type AttrSelector with known attribute names

Replace AttrSelector = Record<string, string> with KnownAttrSelectors
plus a string|boolean index signature, so authors get autocomplete and
type-checking on testTag / focused / clickable / etc. while raw driver
attributes still type-check via the fallback. Boolean state attributes
accept native booleans; goja stringifies them at the marshal boundary.

AccessibilityElement.attrs becomes RawAttrs (typed string-valued shape
of the same canonical names) so element.attrs.testTag autocompletes.

* test(verifier): native boolean selector value matches focused=true

* docs+folio: use native boolean for focused selector and document typed attrs

cmd/sanderling/main.go

@@ -23,6 +23,7 @@ type testOptions struct {
 	duration  time.Duration
 	seed      int64
 	output    string
+	clearData bool
 }
 
 const topUsage = `sanderling is a property-based UI fuzzer for mobile apps.
@@ -51,6 +52,7 @@ func parseTestArgs(args []string, stderr io.Writer) (testOptions, error) {
 	flagSet.DurationVar(&options.duration, "duration", 5*time.Minute, "total test duration")
 	flagSet.Int64Var(&options.seed, "seed", 0, "RNG seed (0 = random)")
 	flagSet.StringVar(&options.output, "output", "./runs", "output directory for traces")
+	flagSet.BoolVar(&options.clearData, "clear-data", false, "clear app data before launching so the run starts from a fresh install")
 	if err := flagSet.Parse(args); err != nil {
 		return testOptions{}, err
 	}