[Core][SM70] Promote DFlash2 NVFP4 prefill route - #417

Purpose

Automatically select the existing lossless QPN2-packed bounded prefill path for the exact Qwen3.8-27B NVFP4 DFlash2 TP4 single-request contract. The default gate requires method=dflash, q7, checkpoint selector top-K 16, PP1/TP4, B1, no DBO/microbatching, and the exact dense layer shapes. Explicit environment overrides remain hard rollbacks; DFlash1, target-only, other speculative methods, TP sizes, and concurrent service retain existing routing.

This PR changes dispatch only. It does not change kernel arithmetic, DFlash2 proposal/verification, sampling, attention, prefix-cache behavior, or the merged tool/structured-output fixes.

BASE_SHA: 62ad1e0

Test Plan

• Focused resolver, DFlash1/DFlash2 selector isolation, exact-shape, zero-copy-layout, and dispatch tests.
• Latest-main TP4 V100 route-hit and cold 32K/64K prefill measurements under the production NVFP4 DFlash2 contract.
• Preserve the merged QPN2 decode/quality contract and verify long-prefill output integrity against retained artifacts.

Test Result

• Focused CPU suite: 5 passed.
• Ruff check/format and git diff --check: passed.
• Four-V100 latest-main route hit: automatic QPN2 M<=8, automatic QPN2-packed prefill M>=1024, fixed-SHA D256 sidecar, direct paged chunked prefill, FP8 E5M2 bridge, and DFlash2 context-only chunk skip all active.
• 32K: 8.0618 s request wall, 4064.62 request-wall tok/s, 4069.25 pure-prefill tok/s.
• 64K: 18.3891 s request wall, 3563.85 request-wall tok/s, 3566.94 pure-prefill tok/s.
• Versus retained pre-closure DFlash2: +30.1% at 32K and +37.7% at 64K. No target-only rerun was performed.
• All six measured requests were uncorrupted and produced token ID 248046 with retained output SHA256 54363ddee68f4a5db81c9d37e5fb738d28f5b67dc7f725ad7333172b1ea157da.
• The unchanged QPN2 decode path was already covered by merged PR [Bugfix][SM70] Audit DFlash2 output quality #346: structured API 24/24, long alternating-prefix state 5/5, three-seed MBPP/HumanEval/LiveCodeBench gates, and target-only/DFlash2 PPL 5.4993116/5.4993622.

Raw artifacts and exact hashes are recorded in docs/design/sm70_dflash2_nvfp4_prefill_promotion.md.