- test_output_clamped now uses inputs that would produce -0.5 without
clamping, so it actually exercises the torch.clamp(..., 0, 1) call.
- test_padding_value_is_one verifies that the channel-alignment logic
pads with 1.0 specifically (not 0.0 or some other value), which is the
semantic guarantee of treating the extra channel as an opaque alpha.
- Add regression tests covering RGB+RGBA, RGBA+RGB, channel gap > 1
(the exact CORE-103 error case), all blend modes with mismatch, and
output value clamping.
- Soften the inline comment to reflect that channel padding is well-
defined for alpha-like extra channels rather than claiming support
for arbitrary channel layouts.
