buffer_cache: no buffer downloads at every fence for Switch 2 titles

Eden copied every GPU-written buffer range back to guest memory at the next fence.
Title 1 signals a fence per compute dispatch, so that was 250-330 MB/s in 13,000-20,000
ranges a second, with the fencing thread holding the buffer cache while it wrote them.
Measured: the native MME's DMA reads never touched those pages (they read query words
written on the CPU side), and with reactive flushing the guest CPU read GPU-written
memory at most 64 times in 200 s. Without them, for Switch 2 titles (native tiling):
fences ~1.1 us instead of ~5, draw setup 6.1 us instead of 6.4-6.7, macro calls 160 ns
instead of 167-170, GPU-thread lock spinning 1.8% (was 5.5%). Regions stay marked GPU
written and are still flushed on demand. EDEN_NX2_BUFFER_DOWNLOADS=0/1 forces either
way. Reactive flushing reads of GPU-written memory are now counted in the log.

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