Fix frame timings and non-monotonic clock issues

[meszmate]

Closes #33

• Use std.time.Timer (monotonic) instead of nanoTimestamp() (wall clock) for frame timing
• Add cross-version sleepNs helper with fallbacks for Zig 0.15 and 0.16+
• Reset timing state on program start to avoid stale deltas
• Fix post-resume frame delta spike