[SP] Fix footstep and view bob speed depending on the frame rate

[Rugaliz]

Problem

PM_Footsteps advances the integer bobCycle with

pm->ps->bobCycle = (int)( old + bobmove * pml.msec ) & 255;

old is an int, so the fraction of bobmove * msec is dropped on every pmove step (0.3 * 6 msec = 1.8 becomes 1). The singleplayer player runs one pmove per rendered frame, so the bob cycle speed, and with it the footstep event cadence and the view/weapon bob, depends on the frame time:

frame time	run (0.4)	walk (0.3)
8 ms (125 fps, the default cap)	-6%	-17%
6 ms (144-166 fps)	-17%	-44%
5 ms (200 fps)	exact	-33%
3 ms	-17%	stops

Swimming (0.15) stops advancing below 7 ms per frame, so EV_SWIM is lost. Higher frame rate caps (high refresh rate displays) hit this directly.

Fix

Advance by floor( bobmove * time ) at the end of the step minus the same expression at its start. The rounding errors of consecutive steps cancel, so for a constant bobmove the cycle advances at exactly the intended rate at any frame rate, and the result does not drift. No state is added: a remainder field in playerState_t would change the save game and network formats.

Applied to both codeJK2/game/bg_pmove.cpp and code/game/bg_pmove.cpp (the two call sites each: swimming and the general case).

Behaviour change

At the default 125 fps running footsteps/bob get about 6% faster and walking about 20% faster, i.e. they now match the nominal bobmove rates; at low frame rates the change is small (e.g. 30 fps: under 2%).

Testing

• Offline simulation of old vs new maths at 3-33 ms frame times (including uneven 6/7 ms frames): the new advance equals bobmove * 1000 per second.
• In game (Jedi Outcast SP, kejim_base, temporary counter that was removed again): running advances 400.0 bob units per moving second at com_maxfps 60, 144 and 200.
• JK2 and JA SP build on Linux; Windows (MinGW/clang cross) builds. JA was only build-tested, not run.

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