port: make ScreenView say what it could not draw; refute 'first-declared paints first'

skipped[] has been tracked and read by nobody since P1, under a comment saying a
silently missing element looks like art. _note_structural prints from inside
ScreenView rather than returning a value for a caller -- routing it through a
caller is exactly what did not happen. Structural skips only; transparent-at-
rest is ordinary animation. Zero found today: a guard, not a fix.

The first version of that scan was a FALSE PASS: screen_view.gd did not parse
(a line inserted at three tabs inside a four-tab block -- the substring assert
matched a shallower indent), so grep counted zero from a dead script. The scan
now counts the summary line as a positive control.

Refutes 'the first-declared element paints first', which would have made the
forced-backdrop rule redundant since all six forced elements are index 0. False
on 8 of 16 screens -- decisively on main_menu, where index 0 is pteff00, painted
LAST, and pteff00 is a measured control.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7FiFFFwbvG2uxdcEh8HyF

port/scripts/boot.gd

@@ -233,6 +233,7 @@ func _ready() -> void:
 	if _menu != null and _sequence.is_empty():
 		_menu_enter(name, true)
 
+	view.structural_skips.clear()
 	var settle := view.settle_time()
 	print("screen %s: %d elements, %d in paint order, design %dx%d, settles at t=%d (%.3f s)" % [
 		name, view.screen["elements"].size(), view.screen["paint_order"].size(),