APSIM discovery via APSIM_EXE_PATH + real-engine tests. Bugfix on 0.10.0's
PEST++ rebuild. Full T1/T2/T3 benchmark battery against real PEST 18 +
pestpp-ies 5.2.16 + native APSIM reproduces the prior baseline exactly (0.0%
on every accuracy metric), confirming no regression from the 15-defect fix.