FsdbReader ffrGetMaxFsdbTag64 returns the view window's end, not the file's

In Verdi FsdbReader (W-2024.09), after ffrLoadSignals over a narrowed view window (ffrSetViewWindow/ffrResetViewWindow), ffrGetMaxFsdbTag64 reports the end of that window rather than the dump's last time. This is true with and without ffrSetLoadNavDBOnlyOnce. Code that clamps the next window to the 'file end' then silently reads [end,end] and gets one change per signal. A cold path that resets the window to the whole range after each read hides the bug, but a navDB-once (warm) reader that never widens hits it on every window after the first, and the result looks like 'navDB-once breaks later windows'. Fix: read the max tag once, before the first windowed load, and cache it. With that fix, navDB-once windows matched the default mode byte for byte for both stored and rebuilt signals at 1% windows.