Clamp left_shift in oapv_dquant() to prevent shift UB

[fkyslov]

Summary

Adds defense-in-depth shift clamping in oapv_dquant() (src/oapv_tq.c):

• Clamp left_shift = oapv_clip3(0, 30, -shift) before left-shifting coef[i] * q_matrix[i] in oapv_dquant().

Testing

• Verified all 24 ctest unit/conformance tests pass with AddressSanitizer and UndefinedBehaviorSanitizer.

[kpchoi]

The clamp is not needed. left_shift is already bounded by the bitstream validation:

• dq_shift is derived as bit_depth - 2 - qp / 6.
• bit_depth is checked to be in [10, 16], and tile_qp in [0, MAX_QUANT(bit_depth)], where MAX_QUANT(bit_depth) = 63 + (bit_depth - 10) * 6.
• So qp / 6 is at most bit_depth, and dq_shift is always in [-2, bit_depth - 2].

In this branch left_shift is therefore always in [0, 2], and oapv_clip3(0, 30, -shift) can never change it. It would only add an operation to the dequantization path, so this change cannot be accepted.