A staging arena is free for reuse once the GPU finished the submission that read it, which is after the next frame
staged: frames that stage in a row need two arenas taking turns. A card of 6.5 GB or less keeps 64 MB of idle arenas
between loads, one arena, so the idle one of the two went at every second present and the frame after made a 64 MB
arena again: 9 to 41 ms of the replay thread each on a Steam Deck, where an arena is system memory the driver clears.

texture_frame now keeps the arenas staged into in the last 2 s, up to 256 MB of them (what a card over 6.5 GB keeps
anyway, so nothing changes there). Two seconds after a load's last upload the 64 MB rule holds again, and the 30 s
one after it. RX_VK_STAGING_BURST_S=<n>: that long, 0: as before.

tests/test_staging_burst.py runs staging() and the give-back loop, cut out of vk_texture.cpp, through frames on a
clock of its own: 20 frames in a row make 2 arenas (11 before), the rule's 256 MB cap, the return to 64 MB.

Assisted-by: Claude Fable 5.2
Co-Authored-By: Claude Fable 5.2 <[email protected]>

@@ -4246,0 +4248,4 @@
+            // made a 64 MB arena again, 9 to 41 ms each on a Steam Deck. RX_VK_STAGING_BURST_S=<n>: that long, 0: as before)
+            static const int64_t burst_s = getenv("RX_VK_STAGING_BURST_S") ? atoll(getenv("RX_VK_STAGING_BURST_S")) : 2;
+            const bool recent = burst_s && std::chrono::steady_clock::now() - it->used_at < std::chrono::seconds(burst_s);
+            const uint64_t allowed = recent ? std::max<uint64_t>(keep, 256ull << 20) : keep;