e0test: check-exchange acceptance stage for edit 6; record RC1 first-light

After the flip leaves counters over threshold, the tool now sends a
CheckRequest (ACKing every status frame) — edit-6 chips must repaint
and re-render the E0 readout; 04000000 marks a pre-rc1 chip. Analysis
records the stock-chip archaeology (stock also leaves 04000000, but the
ungated E0 readout papered over it in period — our fixes exposed it)
and RC1's below-threshold first-light PASS (version+check -> F0).

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

RIOv4_2-ANALYSIS.md

@@ -522,6 +522,17 @@ hooked from the handler's final notify (`$C5E4`): `JSR $C70E` (original
 notify), `JSR $CB7C` (repaint `F0000000`), `JSR $D5F2` (re-render the
 E0 readout, which the edit-5 gate suppresses below threshold), `RTS`.
 Built into **`RIOv4_3rc1.bin`** (with edits 1-2 and 5; sha256
-`dc59bd51cae3…`) — awaiting burn + on-hardware verification, after
-which the image is christened RIO 4.3. Prior generations moved to
-`archive/`.
+`dc59bd51cae3…`); prior generations moved to `archive/`.
+
+Why `04000000` was never seen in period (user-verified by re-slotting
+the ORIGINAL stock chip, 2026-07-19): stock leaves the same stale frame
+after every check, but the ungated E0 readout papers over it at the
+first counter event — `04000000` flashes momentarily, then `E000000x`
+as errors accrue and the reply-latch wedge trips under load. The wedge
+fix plus the E0 threshold gate are what exposed the stale frame.
+
+**RC1 first-light (burned 2026-07-19): PASS on the below-threshold
+path** — a version+check exchange now leaves the display at `F0000000`.
+Remaining for the RIO 4.3 christening: over-threshold check repaint
+(must re-render the E0 readout), `--e0test` regression, mash
+spot-check.