#1339 cuda: add an opt-in SM120 Q8 T8 QKV MMQ path

Beschreibung

Experimental default-off SM120 Q8_0 T8 QKV path. Warm-cache synthetic stage: 20.495 to 14.330 us (30.1% less time). Bounds/capture/lifetime fixtures and sanitizer passed. Fresh 8K/512 model runs: 69.41 to 80.91 tok/s, but tokens and work differ: no isolated request-speedup or quality-equivalence claim. Other shapes use native fallback.

![Results](https://raw.githubusercontent.com/CC-David-CC/Strata-a5500/81f16f4c01e21e4563502831d7231d1e0ba6f542/docs/experiments/dense-t8-main/results.png)