G76 & G33 spindle synchronization bug #3455

Description:

LCNC does not start a threading pass (or start synchronized tapping) at a determinate/consistent location.

Issue first reported on forum by user who noticed that G76 threading passes start point changed depending on spindle speed.
My testing with G33.1 indicated the same issue with tapping: the point where the spindle was 'locked' to the Z-axis changed depending on spindle speed.

It appears to me that despite the manual's description of G33 spindle synchronized motion, the trajectory planner does not calculate the start point based on spindle angular position, desired speed, and accel value.

Background:
Forum thread #1: https://forum.linuxcnc.org/38-general-linuxcnc-questions/54792-threading-index-varies-with-speed?start=0#317088

Thread #2: https://forum.linuxcnc.org/38-general-linuxcnc-questions/55116-g33-1-synchronized-tapping-problem?start=0#319620

Note: disregard discussion of non-mainline component 'caxis.comp' in 2nd thread above.

To save time digging through thread #1, here is some GH history I found on this issue:
https://forum.linuxcnc.org/38-general-linuxcnc-questions/54792-threading-index-varies-with-speed?start=30#317931