Fix capture frame interval handling

[grawis]

Summary

Fix the mismatch between the frame interval reported through
VIDIOC_G_PARM and the actual capture timing.

The previous implementation calculated:

denominator / numerator

For the default 1001/30000 time-per-frame value, this produces about
29.97, which represents frames per second, but was treated as milliseconds.

The new implementation calculates:

frame interval in jiffies = numerator * HZ / denominator

It then subtracts the frame processing time before sleeping, so the actual
capture period follows the configured V4L2 time-per-frame value.

Other changes:

• Rename output_fps to frame_interval because it stores seconds per
  frame, not frames per second.

• Clamp VIDIOC_S_PARM values to the range advertised by
  VIDIOC_ENUM_FRAMEINTERVALS.

• Reuse the same interval definitions for enumeration, initialization,
  setting, reporting, and frame pacing.

Test results

Before:

Frame interval: reported=33.367 ms measured=29.989 ms
error=10.1% [FAIL]

After:

Frame interval: reported=33.367 ms measured=33.997 ms
error=1.9% [PASS]

Additional tests:

25 FPS: reported=40.000 ms measured=41.008 ms error=2.5% [PASS]
YUYV: reported=33.367 ms measured=34.012 ms error=1.9% [PASS]

Fixes #49

---

Summary by cubic

Fixes incorrect frame pacing by computing and applying the V4L2 time-per-frame correctly. Reported and actual intervals now align, reducing timing error from ~10% to ~2%.

• Bug Fixes
  • Compute frame interval in jiffies: numerator * HZ / denominator.
  • Subtract processing time before sleeping to keep the configured period.
  • Rename output_fps to frame_interval (seconds per frame).
  • Clamp VIDIOC_S_PARM to the range from VIDIOC_ENUM_FRAMEINTERVALS.
  • Define shared min/max/default intervals and reuse them for enum/init/get/set/pacing.

^{Written for commit ef4fd9e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}