Hardware e4m3 decode in dec0's tail on the native-FP8 build

make_ptx.py turns the 32 cvt.rn.f16x2.e4m3x2 of rrlite_dec0_4x4_folded into calls of d4r_dec_e4m3x2 (tex_common.h) when D4R_TEX_FP8=1: v_cvt_pk_f32_fp8 and two conversions to f16 instead of ZLUDA's integer lowering (about 25 instructions each; the kernel goes from 4058 to 3418 instructions). The conversion equals ZLUDA's e4m3_to_f16_bits for every code except the NaN codes 0x7f / 0xff, checked over all 65536 code pairs on an RX 9070 XT, and the head never stages those codes (it clamps to +-448 before encoding). Harness frames are byte-identical in the accuracy and the fast set, over a 16-frame moving sequence in the accuracy set. dec0 at 2227x1253 render: 0.43 -> 0.38 ms (accuracy), 0.36 -> 0.30 ms (fast). -DD4R_TEX_SOFT_E4M3 gives ZLUDA's lowering; builds without D4R_TEX_FP8 keep the PTX instruction.

docs/native-kernels.md

@@ -78,6 +78,8 @@ In that build the GEMM activations live in LDS as e4m3 bytes (`SWIN_A8` in `swin
 
 **Texture tails on the native-FP8 path.** The enc0 tail and the dec0 head encode their results with `v_cvt_pk_fp8_f32` on the f32 copies of the f16 values (`e4m3x4_f32` in `tex_common.h`) instead of ZLUDA's integer `e4m3x2`. With the tails' clamps (±2π for enc0, ±448 for dec0, as a median of three) the codes are equal for every non-NaN f16, checked over all 65536 patterns on an RX 9070 XT; a NaN in dec0 now encodes as −448 instead of the NaN code. Harness frames are byte-identical in both sets (LDR and HDR, motion vectors at render and display resolution). dec0: 0.21 → 0.15 ms at 1280×720 render and 0.74 → 0.53 ms at 2560×1440; enc0 changes by 1–2% (its time is ZLUDA's compile of the feature code). `-DD4R_TEX_SOFT_E4M3` (`D4R_TEX_CFLAGS`) keeps `e4m3x2`.
 
+dec0's tail decodes the staged codes with the hardware conversion too. On this build `make_ptx.py` turns its 32 `cvt.rn.f16x2.e4m3x2` into calls of `d4r_dec_e4m3x2` (`tex_common.h`: `v_cvt_pk_f32_fp8` and two conversions to f16), where ZLUDA lowers each one to about 25 integer instructions. The conversion equals ZLUDA's `e4m3_to_f16_bits` for every code except the NaN codes `0x7f` / `0xff` (all 65536 code pairs checked on an RX 9070 XT), and the head never stages those: it clamps to ±448 before it encodes. Harness frames are byte-identical in both sets, over a 16-frame moving sequence in the accuracy set. dec0 at 2227×1253 render: 0.43 → 0.38 ms in the accuracy set and 0.36 → 0.30 ms in the fast set. With `-DD4R_TEX_SOFT_E4M3` the function is ZLUDA's lowering; builds without `D4R_TEX_FP8` keep the PTX instruction.
+
 RX 9070 XT, accuracy set, D3D12 harness at 3840×2160 output, median GPU ms per frame (`D4R_CUDA_KERNEL_PROFILE`, frames 81–200), previous build → this one:
 
 | Render | enc1 | dec1 | enc3 tube (6 launches) | whole evaluation |