Docs: record E0-threshold bench PASS (E0000105) + corrected counter semantics

The display observation recalibrates the counters: $3184 = give-up
cycles (once per exhausted retry sequence, not per retransmit), $3185 =
teardowns of any reply cycle needing at least one retransmission — the
counter behind a lone E0000001. Also recorded in the error-handling
inventory: 5 retransmits per cycle, reply-await arms only after the
complete frame, NAK forces exactly one counted retransmit, late ACKs
cannot rescue a cycle. (Previous commit's doc edits had silently missed
their anchors; applied properly this time.)

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

display-board-1408.md

@@ -99,9 +99,12 @@ also the RIO's **error annunciator**:
 
 - **`E0` serial-diagnostics readout** (`$D5F2`) — `E0xxyyzz` where
   `xx` = TX-ring overflow count (`$3187`, outbound byte dropped because
-  the ring was full, `$D66A`), `yy` = reply-retransmit count (`$3184`,
-  the game failed to ACK a reply, `$D96A`), `zz` = reply-teardown/give-up
-  count (`$3185`, `$DA2F`). Each increment site immediately re-renders,
+  the ring was full, `$D66A`), `yy` = give-up count (`$3184`, `$D96A` —
+  reply cycles whose 5 timeout retransmits all went unACKed), `zz` =
+  reply-teardown count (`$3185`, `$DA2F` — any reply cycle that needed
+  at least one retransmission; the counter behind a lone `E0000001`).
+  Semantics bench-confirmed 2026-07-19: display read `E0000105` after
+  one give-up + five teardowns. Each increment site immediately re-renders,
   so the display live-updates as serial trouble accumulates. If a
   cockpit's display reads `E0…` instead of `F0000000`, the serial link
   has been dropping or retrying. Note there is **no threshold in stock