port: count the fallbacks instead of inspecting them; black_hold's absence is now audible

The Decoder sharpened the sweep in a way that invalidates part of how I ran it: an
in-range fallback cannot be caught by inspecting output, because the output looks
exactly like the true case -- the only way to know is to count how often it fires.
My sweep classified defaults as identity or sentinel by inspection, which is
precisely the method that cannot see this.

Counted: rotation_deg -> 0 fires 0 times in 866 keyframes and 178 rest poses, and
ramp is present in authored/. So rotation is read, not invented -- the same
conclusion they reached for design size, reachable only by counting.

The count exposed one I had waved through twice: black_hold_units defaults to 0.0
and its authored value IS 0, so deleting the entry would be invisible -- same
behaviour, no error, and the reasoning in black_hold_why (four measured gaps, why
0 over the better-fitting 4 or 6, the tripwire) silently stops applying. Fixed the
same way as exit_ramp_units: fallback is -1.0 and an absent key raises an error
naming what was lost.

The control is the demonstration: key present 0 errors, key deleted 1 error, and
the render byte-identical either way. No output inspection could have detected the
deletion.

Does not change the value: still 0, still wrong by 4-6 units on three of four
measured transitions, still no rule. Only its absence is now audible.

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

port/scripts/boot.gd

@@ -206,7 +206,23 @@ func _ready() -> void:
 	# Negative means "not supplied": ScreenView then declines to invent a duration
 	# and says so, rather than making one up. See ScreenView.exit_ramp_units.
 	view.exit_ramp_units = float(timing.get("exit_ramp_units", -1.0))
-	_black_hold = float(timing.get("black_hold_units", 0.0))
+	# ⚠️ THE FALLBACK IS -1.0, NOT 0.0, EVEN THOUGH THE AUTHORED VALUE IS 0.
+	#
+	# This is the `exit_ramp_units` shape in waiting: a default that EQUALS the
+	# authored value makes deleting the authored entry invisible -- same
+	# behaviour, no error, and the reasoning in `black_hold_why` (four measured
+	# gaps, why 0 rather than the best-fitting 4 or 6, and the tripwire for
+	# revisiting it) silently stops applying to anything.
+	#
+	# The Decoder's sharpening is what surfaced it: an IN-RANGE fallback cannot
+	# be caught by inspecting output, because the output looks exactly like the
+	# true case. 0 is a legitimate hold. So the absence is made loud instead.
+	_black_hold = float(timing.get("black_hold_units", -1.0))
+	if _black_hold < 0.0:
+		push_error("authored/timing.json has no black_hold_units. Using 0, which is "
+			+ "what it said -- but the REASONING for 0 lived there and is now gone. "
+			+ "See black_hold_why in git history before trusting this transition.")
+		_black_hold = 0.0
 	# Which focus records draw unconditionally and loop. Kept out of ScreenView's
 	# own logic on purpose -- see `looping_focus` there for the census that says
 	# this cannot be a rule.