FsdbReader: time-ranged loads via view window (Set once, then Reset)

In Verdi's FsdbReader C++ API, setting a view window before ffrLoadSignals makes the load hold only the changes in [start, end] plus one synthetic change at start carrying the value in effect there, so a [t, t] window is a ~2-18 ms cold value read and a quarter-range load costs about a quarter of a full load. Gotchas found by measurement: only the first call may be ffrSetViewWindow; later Set calls are silently ignored and you must use ffrResetViewWindow (also to widen back to the whole file before other loads). A window past the file's last change loads nothing, so clamp it to ffrGetMaxFsdbTag64, and ffrGotoXTag past the last loaded change fails; fall back to the handle's max xtag. ffrGetVCCount is 0 before a load (no free size estimate) and over-counts after a windowed load. Splitting a wide-signal walk across processes only scales once each slice uses its own window; without it every slice pays the full load.