Merge main: Tests/Manual revival, functional fixes

Merges 71 commits from main. Headline content: the Tests/Manual
integration test suite revived and reorganized after long disuse
(renames, new applications, updated cibs), restoring a large
integration validation surface. Functional fixes include replacing
unload-based termination with pagehide (back/forward cache
compatibility) and hardening CPTextField's input-element handling.

Conflict resolutions (local versions retained):

- AppKit/CPTextField.j: main's input-element logic carried, with this
  branch's preprocessor-removal sweep re-applied on top. Main's
  conditionals are re-expressed as runtime CPDOMAvailable guards; the
  _inputElement method-definition gate is retained.

- AppKit/Platform/CPPlatform.j: the DOM platform implementation stays
  inlined under the PLATFORM(DOM) gate. The split layout breaks the
  legacy build: the include fragment is compiled standalone by the
  source walk and cannot resolve its superclass. Main's pagehide
  termination fix is ported into the inlined section; the restored
  AppKit/Platform/DOM/CPPlatform.j fragment is deleted.

Verified: legacy build green through Foundation, AppKit and Tools;
Tests/Manual suite passing.

AppKit/CPTextView/CPTextView.j

@@ -3298,7 +3298,7 @@ var _CPCopyPlaceholder = '-';
         var currentFirstResponder = [[CPApp keyWindow] firstResponder];
 
         // Can we accept richtext? Then this is our preference (fixme: shift key to force plain text paste)
-        if ([currentFirstResponder isRichText])
+        if ([currentFirstResponder respondsToSelector:@selector(isRichText)] && [currentFirstResponder isRichText])
         {
             var richtext = nativeClipboard.getData('text/rtf');