feat(ttl): Read the FDR counters in every Script B soak round

Script B runs blanton_fdr.sh rdcnt after the Tray ID stress in each
round, so the FDR counters started before the loop are logged through
the soak, not only at Script C. The call is not gated by SWB_UNIT0 /
SWB_UNIT1. Recorded under V1.2.6 in Script A's history and the release
note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0131orciRKrejchsuDk6UpXk

docs/release-notes/v1.2.6.md

@@ -10,7 +10,7 @@
 
 **FDR runs alongside the traffic test in Script A, B and C**
 * **Script A** wraps its traffic baseline: `fdr start` after `init`, `fdr clear` after `clear`, `fdr rdcnt` after the run, `fdr stop` after the traffic stop. The traffic run is now **40 s instead of 15 s**, so the counters are read after the first 30 s interval has been published — the same rule as `prbsstat Ber`.
-* **Script B** starts and clears FDR with the traffic setup, before the soak loop — the counters run for the whole soak.
+* **Script B** starts and clears FDR with the traffic setup, before the soak loop — the counters run for the whole soak — and reads them (`rdcnt`) in every soak round, after the Tray ID stress.
 * **Script C** reads them (`rdcnt`) before stopping the traffic and stops FDR after it — the closing read of the counters Script B started.
 
 **Script A turns off the unused lanes before the BER baseline**
@@ -32,6 +32,7 @@ grep -rlU $'\r' ~/Blanton_Script    # expect no output
 * **`chmod +x` after deploying.** `blanton_fdr.sh`, `unused_lanes.sh` and `retimer_lanes.sh` are all called by path; `unused_lanes.sh` also refuses to start if `retimer_lanes.sh` is not executable.
 * **`blanton_fdr.sh stop` sends `fdrstat STOp` — that keyword is inferred from `prbsstat STOp` and has not been checked on the bench.** `bcmcmd` exits 0 even when the BCM shell rejects a command, so if it is wrong, FDR keeps running and only the log shows it. Check the first run.
 * **FDR only runs when both switch units are enabled** (`SWB_UNIT0 = SWB_UNIT1 = 1`). The single-unit branches have no FDR, and Script A's single-unit traffic run is still 15 s.
+* **Script B's per-round `fdr rdcnt` is not gated by the unit settings** — with one unit enabled, FDR was never started and each round just logs whatever `fdrstat Counters` returns.
 * **Nothing turns the unused lanes back on.** Script C has no `unused_lanes.sh on`; run it by hand when the DUT is needed in its normal state. Whether a reboot clears it has not been checked.
 * **`unused_lanes.sh off` is not gated by `SWB_UNIT0` / `SWB_UNIT1`** — it always acts on both boards.
 * **`blanton_ber.sh` (Script A / C) still uses `p=3` and `sleep 30`**; see v1.2.5.