ImageFileReader accepts invalid frame indices #449

Describe the bug

ImageFileReader.read_frame_raw() uses index > number_of_frames as its bounds check. This allows negative Python indexing and also misses the first invalid upper-bound value, index == number_of_frames.

To reproduce

Open any multiframe image and call:

reader.read_frame_raw(-1)
reader.read_frame_raw(reader.number_of_frames)

Current behavior: the negative index accesses the last offset-table entry, while the upper-bound case fails later through incidental indexing or I/O behavior.

Expected behavior

Valid indices should be exactly 0 <= index < number_of_frames. Invalid indices should raise a clear ValueError before indexing the offset table or seeking into pixel data. read_frame() should inherit the same contract.

Scope

Correct the bounds predicate, document the exception, and add lower- and upper-bound regressions.

Process note

I am filing this issue before the replacement pull request in accordance with the developer guide's issue-first recommendation. PR #434 was opened before that preflight and will be superseded.