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/test_run.go

@@ -17,5 +17,6 @@ func runTestPipeline(ctx context.Context, options testOptions, stdout io.Writer)
 		Duration:  options.duration,
 		Seed:      options.seed,
 		Output:    options.output,
+		ClearData: options.clearData,
 	}, stdout)
 }