Fix test image projection formula in lab11.m

[tsainez]

What:
Fixed the test coefficients computation formula in matlab/lab11.m and resolved the outstanding TODO.

Why:
The previous logic overwrote the original test image shape array (img_test) with a 1D vector during mean subtraction before applying the projection. The new approach uses a single matrix multiplication line coef_test = U' * (img_test(:) - x_bar); that safely calculates the projection without destroying the test image variable.

How:

• Replaced the two lines handling mean subtraction and projection calculation with a single inline transformation.
• Validated via mocked structural test datasets to ensure mathematical parity and correct dimensions are preserved.
• Maintained backward compatibility with existing Eigenfaces classification implementation (accuracy scores intact).

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PR created automatically by Jules for task 11243763330961440179 started by @tsainez

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