Allow DC coefficient differences beyond 16 bits and validate the reconstructed value

[kpchoi]

Fixes #254.

DC coefficients are in [-32768, 32767], so the difference between two neighboring blocks can legally reach 65535. RFC 9924 places the range constraint on the reconstructed coefficient, not on the difference, and the encoder writes such differences without any bound. The decoder rejected them, so it failed to decode conforming streams produced by its own encoder — 12-bit at QP 0-4, and any bit depth with a custom quantization matrix.

oapvd_vlc_dc_coef(): raise the abs_dc_diff limit from 32767 to 65535, and apply the sign with oapv_set_sign() instead of the misleadingly named oapv_set_sign16() (both operate on int, so this is a rename, not a behavior change).

dec_block(): compute the predicted DC in int and validate it against MIN_TX_VAL/MAX_TX_VAL before storing to the s16 coefficient buffer. This is where the range constraint belongs, and it also closes a pre-existing overflow: the old code summed two int values into an s16 with no check, so a value up to 65534 could wrap silently.

[cpncf]

LGTM