feat(pflash): improve AMD defaults and enable rocWMMA

[Graffioh]

Summary

• use a HIP-specific sparse-alpha default of 0.95 while preserving user overrides and logging the effective value
• seed fresh PFlash sessions from the resolved keep ratio, including clamping and sticky-session behavior
• auto-enable rocWMMA Phase 2 kernels for fresh HIP builds when headers are available, while preserving explicit OFF and warning on q8 fallback

Validation

Tested on the local Strix Halo gfx1151 GPU pinned with HIP_VISIBLE_DEVICES=1.

• fresh HIP configure: rocWMMA detected and enabled by default
• explicit -DDFLASH27B_P2_USE_ROCWMMA=OFF: fallback remains disabled as requested
• affected server and flashprefill targets compile and link with -j$(nproc)
• fresh-session configured-ratio test passed
• clamped/sticky session-seed test passed
• HIP flashprefill kernel test passed on gfx1151
• git diff --check passed

A full all-target HIP build reaches the unrelated existing test_rms_norm_hip.cpp:50 compatibility failure (cudaErrorNoDevice is not defined for this HIP build). The affected targets compile and link successfully.