docs: replace route 7's unfalsifiable gate criterion with a positive assertion (#319)

The original criterion — "the session counts ONLY if cause=propagate lines
appear" — cannot fail in the presence of the bug it exists to catch. #319
makes a player-parented child emit no probe line at all, so the defect's
signature is ABSENCE, which that wording reads as "not exercised" rather than
"broken". Two captured gate logs contain #319 and neither flags it; the second
was run specifically to thicken this gate and still missed it.

Replaced with a positive assertion: read the equipped child's FullCellId and
require it to EQUAL the parent's after a crossing — a zero child cell is a
failure, not a silence. Probe-line volume drops to a secondary check. And the
gate must now be run with a PLAYER parent as well as a creature parent, since
#319 exists precisely because every probe-firing parent in both logs was
instance-sequence 0 and the sole player parent was the sole failure.

The rule this generalises, added to the handoff's process findings: a gate
whose failure mode is indistinguishable from a not-run manufactures
confidence. Counting evidence-of-success is not the same as asserting the
property; only the latter can fail.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/research/2026-08-05-c4-closeout-handoff.md

@@ -97,9 +97,31 @@ traffic); this is the one purely visual gate.
 6. Portal recall while equipped — equipment present and following after
    arrival.
 
-**Pass criterion: the session counts ONLY if `[child-cell]` lines with
-`cause=propagate` appear during step 2**, child cell always equal to the
-player's. Regressions: weapon drawn at the world origin or its last ground
+**Pass criterion — CORRECTED 2026-08-05, the original was unfalsifiable.**
+
+The original read: *"the session counts ONLY if `[child-cell]` lines with
+`cause=propagate` appear during step 2."* **That criterion cannot fail in the
+presence of the bug it exists to catch.** #319 makes a player-parented child
+emit NO probe line at all, so the defect's signature is ABSENCE — which the
+old wording reads as "you didn't exercise it" rather than "it is broken." Two
+captured gate logs (`c4-gates.log`, `c5-gates.log`) contain #319 and neither
+flags it; the second was even run specifically to thicken this gate.
+
+A gate whose failure mode is indistinguishable from a not-run manufactures
+confidence. Replace it with a POSITIVE assertion:
+
+1. **Assert the equipped child's `FullCellId` EQUALS the parent's** after a
+   landblock crossing — read it, do not infer it from probe volume. A zero
+   child cell is a FAILURE, not a silence.
+2. Then, and only as a secondary check, expect `cause=propagate` counts in
+   double digits across several crossings.
+3. Run it with a **player** parent AND a **creature** parent. #319 exists
+   precisely because every probe-firing parent in both captured logs was
+   `0x7…`/`0x8…` (instance sequence 0) and the sole `0x5…` player parent was
+   the sole failure. A gate that only ever sees sequence-0 parents is blind to
+   the entire player class.
+
+Regressions: weapon drawn at the world origin or its last ground
 position; invisible while equipped; **left behind at a landblock boundary**
 (the D4 demotion's specific risk); invisible-but-solid at a former position
 (#184); child culled while the parent is visible or vice versa. The headless
@@ -379,7 +401,9 @@ load-bearing observation: before the round-2 R3 fix the field read
 
 **Route 7 (child cell) — PASS, but THIN.** 17 `[child-cell]` lines:
 13 `cause=attach`, 3 `cause=delete`, **1 `cause=propagate`**. The stated pass
-criterion (at least one `cause=propagate`) is met, so the gate passes — but
+criterion (at least one `cause=propagate`) was met, so the gate was recorded
+as passing — but see #319: that criterion was UNFALSIFIABLE and this very log
+contains an undetected defect. Corrected above. The remaining note stands:
 propagation across a parent cell crossing is the slice's entire purpose, and
 one sample proves the path executes rather than that it is robust across
 repeated crossings. A future session should run several equipped landblock