Fix cumulative argmax/argmin returning window-local indices (GH#11336)

[C1-BA-B1-F3]

Problem

When calling argmax() or argmin() on a cumulative rolling window, the result returns window-local indices (positions within the NaN-padded window) instead of absolute indices in the original array.

For example, with data [1, 2, 1.5, 3.5, 4]:

• cumulative('time').argmax() returns [4, 4, 3, 4, 4]
• Expected: [0, 1, 1, 3, 4]

Root Cause

Cumulative rolling constructs a window with NaN padding on the left. argmax/argmin returns the position within this padded window, not the absolute index in the original array.

For example, at position 2:

• Window: [nan, nan, 1.0, 2.0, 1.5]
• argmax returns 3 (position of 2.0 in the window)
• But the absolute index should be 1 (position of 2.0 in the original array)

Fix

The fix detects cumulative rolling (window size == dimension size, not centered) and adjusts the returned indices by subtracting the NaN padding offset:

absolute = local - (window_size - 1 - position)

Regression Tests

• test_cumulative_argminmax: Tests both argmax and argmin with 1D data
• test_cumulative_argmax_2d: Tests argmax with 2D data

Closes #11336