hud_strafehelper.c bugfixes, refactors, cleanup

[taysta]

cleanup:

• Add const qualifiers to function parameters and local variables where applicable
• Remove redundant parentheses
• Reduce variable scope and move declarations closer to usage
• Replace unnecessary temporaries and redundant else blocks
• Use sqrtf, acosf, asinf, floorf consistently for float math
• Minor formatting and readability improvements
• Remove unused variables and dead code

correctness:

• Initialize conditional variables in DF_StrafeHelper
• Remove duplicate airaccelerate assignment in DF_SetPhysics
• Early return on division by zero in DF_GetCmdScale
• Initialize scale in DF_GetWishspeed to avoid uninitialized use

fix:

• Fix keys sticking in movement keys overlay when in spec/demo playback