Previously the array sim only excited a single inner element
(DRIVEN_X / DRIVEN_Y). Adds DRIVEN_PORTS env var accepting a
comma/semicolon-separated list of "i,j" pairs that are all excited
in-phase with equal amplitude — models a perfect 1:N corporate
splitter feeding an N-patch sub-array.
Example: DRIVEN_PORTS="0,0;1,0;2,0;3,0;0,1;1,1;2,1;3,1" excites a
4-cols × 2-rows sub-array anchored in the corner.
S-parameter post-processing reframed for the multi-driven case:
each excited port reports active-S11 (uf_ref/uf_inc with all driven
ports active); each non-excited port reports S relative to a
representative driven port's incident wave (all driven ports have
equal amplitude so any reference works).
Backwards-compatible: empty DRIVEN_PORTS reverts to single-port
DRIVEN_X / DRIVEN_Y behaviour.
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