feat(ttl): Drive traffic and port wait from SWB_UNIT0/SWB_UNIT1

The bench is not always fully populated. config.ttl now declares which
switch units exist:

    SWB_UNIT0 = 1   ; this DUT has switch unit 0
    SWB_UNIT1 = 1   ; this DUT has switch unit 1

Two values are derived there rather than repeating the same test in
three scripts: swb_any (0 = no unit at all) and swb_opt, the suffix
appended to each blanton_traffic_linespeed call -- "" for both units so
the tool's own TL_UNITS="0 1" applies, " -u 0" or " -u 1" for a single
one.

Script A, B and C build their traffic commands with sprintf2 and the
suffix, wrapped in `if swb_any = 1`. A half-populated DUT no longer
issues bcmcmd against an absent unit, and a DUT with no switch board
skips the traffic stage outright instead of filling the log with
failures.

wait_init.ttl reads the same two flags instead of its own copies, so the
wait and the traffic blocks cannot disagree about what is installed.

Script A -> V1.0.5.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RetWKFZFG1ZHcitQAyhwM

src/Script_ABC_Blanton/utils/wait_init.ttl

@@ -7,11 +7,12 @@
 ; Wait until every switch unit ENABLED below reports more than WT_MIN ports up:
 ;     bcmcmd -n <u> -c ps | grep -w up | wc -l
 ;
-; The DUT is not always fully populated, so declare what this bench has:
-;     both units  -> WT_UNIT0 = 1 , WT_UNIT1 = 1
-;     unit 0 only -> WT_UNIT0 = 1 , WT_UNIT1 = 0
-;     unit 1 only -> WT_UNIT0 = 0 , WT_UNIT1 = 1
-;     no unit     -> WT_UNIT0 = 0 , WT_UNIT1 = 0   (bypass, returns immediately)
+; The DUT is not always fully populated. Which units exist is declared ONCE in
+; config.ttl, so the traffic blocks in Script A/B/C and this wait agree:
+;     both units  -> SWB_UNIT0 = 1 , SWB_UNIT1 = 1
+;     unit 0 only -> SWB_UNIT0 = 1 , SWB_UNIT1 = 0
+;     unit 1 only -> SWB_UNIT0 = 0 , SWB_UNIT1 = 1
+;     no unit     -> SWB_UNIT0 = 0 , SWB_UNIT1 = 0   (bypass, returns immediately)
 ;
 ; Enter : prompt of the previous command is NOT consumed.
 ; Exit  : a command has been sent, prompt NOT consumed (caller does the wait).
@@ -30,10 +31,11 @@
 ;   V3.0.0  2026-08-21  Add WT_UNIT0 / WT_UNIT1 so a partly populated DUT can
 ;                       be declared: wait on the enabled units only, and bypass
 ;                       entirely when neither is set.
+;   V3.1.0  2026-08-21  Take the population from SWB_UNIT0 / SWB_UNIT1 in
+;                       config.ttl instead of local copies, so this wait and the
+;                       traffic blocks cannot drift apart.
 ; =============================================================================
-; ---- edit these for the bench ----------------------------------------------
-WT_UNIT0    = 1       ; 1 = this DUT has switch unit 0, wait for it ; 0 = skip
-WT_UNIT1    = 1       ; 1 = this DUT has switch unit 1, wait for it ; 0 = skip
+; ---- which units to wait for: SWB_UNIT0 / SWB_UNIT1, set in config.ttl ------
 WT_MIN      = 216     ; an enabled unit must report MORE than this many ports up
 WT_INTERVAL = 10      ; seconds between polls
 ; -----------------------------------------------------------------------------
@@ -42,8 +44,8 @@ timeout = 60          ; per-wait cap; bcmcmd ps is not instant
 wait prompt_sonic_root
 
 ; nothing declared -> bypass
-if WT_UNIT0 = 0 then
-    if WT_UNIT1 = 0 then
+if SWB_UNIT0 = 0 then
+    if SWB_UNIT1 = 0 then
         goto NO_UNITS
     endif
 endif
@@ -51,7 +53,7 @@ endif
 :WAIT_PORTS
 wt_ok = 1
 
-if WT_UNIT0 = 1 then
+if SWB_UNIT0 = 1 then
     wt_up0 = 0
     sendln "echo PORTS0=$(bcmcmd -n 0 -c ps | grep -w up | wc -l)"
     waitregex "PORTS0=[ ]*([0-9]+)"
@@ -64,7 +66,7 @@ if WT_UNIT0 = 1 then
     endif
 endif
 
-if WT_UNIT1 = 1 then
+if SWB_UNIT1 = 1 then
     wt_up1 = 0
     sendln "echo PORTS1=$(bcmcmd -n 1 -c ps | grep -w up | wc -l)"
     waitregex "PORTS1=[ ]*([0-9]+)"