Match browser duration rounding to the Python frame grid

[ethanfel]

The browser could round requested durations down because JavaScript remainder can be negative. For example, 1 second became 22 frames in Studio while the Python compiler generated 39. Seconds-to-exact-frame conversion could therefore shorten a scene, and requests just beyond the maximum duration could be accepted incorrectly.

Normalize the browser remainder before adding padding and refresh the Plan core import cache keys. Existing exact-frame inputs and Python compilation are unchanged.

Validation:

• Compared 7,829 durations against the actual Python helper, including fractional-frame requests, floating-point boundaries and maximum-length rejection.
• Compared six compiled Plans with browser raw/delivered timing and checked seconds-to-frame conversion; no project files were written.
• Existing Plan editor, Studio, scene-duplication and chapter-resolution JavaScript regressions passed.

The tests use CPU-only native helpers with ComfyUI import stubs. No production project or GPU execution was used.