Compute a form control's captured attributes and text in one pass

[morisil]

Found in the review of the capture-live-form-state branch.

SnapshotDom.capturedAttributeMap and replacementText (CapturePage.kt) each rebuild formState() and attributeMap() for the same element, so every input / textarea / select / option does this work twice (and isSecretControl runs twice per textarea).
Both producers also wire two shared functions with identical (name, attributes, state) parameters.

Fix: a single shared captureFormControl(name, attributes, state) in FormControls.kt returning both the attributes and the replacement text, called once in walkElement / flowElement.