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

docs/port/DECISIONS.md

@@ -9,7 +9,7 @@ dies, which is what this file is for.
 
 <!-- INDEX: generated by tools/port/index-decisions -- do not hand-edit -->
 
-171 sections. Search this before re-deriving anything.
+172 sections. Search this before re-deriving anything.
 
 * [P0 — the exporter, 2026-08-28](#p0--the-exporter-2026-08-28)
 * [P1 — Godot draws the screen, 2026-08-28](#p1--godot-draws-the-screen-2026-08-28)
@@ -182,6 +182,7 @@ dies, which is what this file is for.
 * [🔴 CORRECTION: my branch *is* the stale era, and the reference binary was never the workspace build](#correction-my-branch-is-the-stale-era-and-the-reference-binary-was-never-the-workspace-build)
 * [`exit_ramp_units`: the refuted constant was living in a default](#exit_ramp_units-the-refuted-constant-was-living-in-a-default)
 * [Auditing the whole tree for "a deleted value that something still supplies"](#auditing-the-whole-tree-for-a-deleted-value-that-something-still-supplies)
+* [Counting the fallbacks instead of inspecting them — and one I had misjudged](#counting-the-fallbacks-instead-of-inspecting-them--and-one-i-had-misjudged)
 
 <!-- /INDEX -->
 ## P0 — the exporter, 2026-08-28
@@ -9805,3 +9806,48 @@ opening the definition, which took thirty seconds and is the whole difference.
 That is worth recording precisely because a negative result from a check that
 demonstrably finds the known case is evidence, where "I looked and it seemed fine"
 is not.
+
+## Counting the fallbacks instead of inspecting them — and one I had misjudged
+
+The Decoder sharpened my 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:
+
+| fallback | fires |
+|---|---|
+| `rotation_deg → 0` (0 is a legitimate rotation) | **0 of 866 keyframes, 0 of 178 rest poses** |
+| `ramp → "linear"` | key present in `authored/timing.json` |
+
+✅ So rotation is **read, not invented** — the same conclusion they reached for
+design size, and reachable only by counting.
+
+### 🔴 The count exposed one I had waved through
+
+`black_hold_units` defaults to `0.0` **and its authored value is 0**. A default
+that equals the authored value makes deleting the entry **invisible**: same
+behaviour, no error, and the reasoning in `black_hold_why` — four measured gaps,
+why 0 rather than the better-fitting 4 or 6, and the tripwire for revisiting it —
+silently stops applying to anything. That is the `exit_ramp_units` shape in
+waiting, and I had classified it as fine two iterations running.
+
+✅ Fixed the same way: the fallback is now `-1.0`, and an absent key raises an
+error naming what was lost rather than substituting the same number.
+
+**The control is the demonstration:**
+
+| | errors | render |
+|---|---|---|
+| key present | 0 | — |
+| key **deleted** | **1** | **byte-identical** |
+
+📌 The render being identical either way *is* the finding. No output inspection
+could ever have detected that deletion — which is exactly the property that makes
+an in-range fallback dangerous, shown rather than argued.
+
+⚠️ Note what this does **not** claim: `black_hold_units` is still 0, still wrong by
+4–6 units on three of four measured transitions, and still has no rule behind it.
+What changed is only that its *absence* is now audible.

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.