Detecting VCS -xprop merge mode from simv.daidir, not the FSDB

An FSDB records nothing about VCS X-propagation. To recover it, read <simv>.daidir/vcs_rebuild, which holds the full vcs command line including every -xprop= argument. A bare -xprop means tmerge, and -xprop=<cfg> means reading the last merge= statement in that config. -kdb builds keep a copy at simv.daidir/xprop.cfg, and xprop.log (file:line YES/NO per statement) appears only in xprop builds. The compile log is unreliable because an up-to-date rebuild just prints 'design hasn't changed'. -xprop=flowctrl is not a merge mode: it only extends instrumentation to loops with break/continue/return. Measured on W-2024.09 with s=x: tmerge gives a bitwise merge of all candidate branches (case with sel=2'b0x merges items 00 and 01 but not default; a partial write merges with the old value), xmerge gives all-x, and an ambiguous 0->x clock edge gives merge(d,q) under tmerge.