Skip IST/STX cascade for intra-luma zero-quant primary

[yeqing-wu]

Extend the intra-luma IST/STX pre-skip and post-trellis gates to cover eob == 0 in addition to the existing eob == 1 (DC-only) case. The eob == 0 branch kills candidates only when stx > 0, so a non-DCT primary with stx == 0 that quantizes to all-zero can still win via skip coding.

Add a pre-quant L-infinity gate that predicts primary FP-quant eob = 0 directly from the post-primary coefficients (max |coeff| below the FP quantizer kill threshold min(dq0, dq1) >> (4 + log_scale)). When fired, av2_quant + av2_optimize_b + cost_coeffs are skipped and the same eob_found / DCT_DCT propagation as the post-quant gate is applied. Scoped to intra Y, stx == 0, non-DC-only, IST-enabled, non-QM blocks.

Gated behind a new tx_sf.prune_intra_ist_stx_by_zero_eob flag, off by default and enabled at speed >= 1. Encoding at speed 0 is unchanged from the baseline.

STATS_CHANGED

Anchor: commit dd84ac9
Speed 1 (cpu-used=1): FG16 CTC (33 frames, RA)

  +------------+-------+-------+-------+-------+------+------+
  | Class      |     Y |    Cb |    Cr |  wAvg | Enc% | Dec% |
  +------------+-------+-------+-------+-------+------+------+
  | A1         |  0.06 | -0.20 | -0.34 |  0.02 |   98 |  100 |
  | A2         |  0.09 | -0.18 | -0.12 |  0.07 |   97 |  100 |
  | A3         |  0.05 |  0.58 |  0.48 |  0.07 |   96 |   99 |
  | A4         |  0.09 |  0.22 | -3.02 | -0.01 |   97 |   99 |
  | A5         | -0.12 | -2.35 |  2.22 | -0.12 |   97 |  101 |
  | B1         |  0.14 | -0.94 | -0.28 |  0.09 |   97 |  101 |
  | B2         | -0.48 | -0.71 |  0.30 | -0.46 |   97 |   97 |
  | Avg w/o B2 |  0.07 | -0.32 | -0.24 |  0.04 |   97 |  100 |
  +------------+-------+-------+-------+-------+------+------+

[yeqing-wu]

Hello all,

Could you please review this PR?

Thanks
Yeqing