perf: store Qwen Image 2.1 prefix cache as F16 with flash attention

[RChang06]

Summary

See #2040

• Store the Qwen Image 2.1 prefix K/V cache as F16 when flash attention is on, since flash attention converts K and V to F16 before the kernel anyway. Output is unchanged, cache memory is halved. Without flash attention, with sage attention, or with --attn-scale, it stays FP32.
• Store, reuse-path concat, and validator all take the type from one prefix_cache_type() helper. the validator now checks the expected type instead of FP32 only.

Additional Information

All outputs compared by SHA-256 of the decoded RGBA pixels at a fixed seed. Every base/PR pair matched.

RTX 4090 24 GB, qwen_image_2.1-Q8_0.gguf, Qwen3VL-8B-Instruct-Q4_K_M.gguf, mmproj-Qwen3VL-8B-Instruct-F16.gguf, 1024x1024, euler, seed 42, --diffusion-fa --offload-to-cpu:

test	master (FP32 cache)	this PR (F16 cache)	pixels
1 reference, 20 steps	cached, 1.20 s/step	cached, 1.20 s/step	identical
2 references, 20 steps	does not fit, falls back, 4.13 s/step, 98.5 s	cached, 1.36 s/step, 36.5 s	identical
2 references, cfg 4 + negative, --max-vram 16	does not fit, falls back, 8.20 s/step	first condition cached, second falls back, 8.29 s/step	identical
no flash attention (FP32 kept)	fallback	fallback	identical
--sage-attn (FP32 kept)	fallback	fallback	identical
--attn-scale 2 (FP32 kept)	fallback	fallback	identical
--model-args qwen_image_2_1_prefix_cache=false		same as uncached master	identical

For two 1024x1024 references (8227-token prefix) the FP32 cache allocation asked for 16967 MB and failed; the F16 one needs 8730 MB and fits next to the weights.

Quadro T2000 4 GB, qwen_image_2.1-Q4_K.gguf, one 512x512 reference (assets/qwen/qwen_image_2.1.png), prompt change 'qwen2.1.cpp' to 'sd.cpp', 8 steps, euler, seed 42, --diffusion-fa --offload-to-cpu:

	master	this PR
cache	FP32, does not fit, falls back after a failed prefill	F16, cached (1052 tokens)
per step	25.0 s	12.0 s
sampling, 8 steps	236 s	111 s
pixel hash	db04ec5b…	db04ec5b…

master (left) vs this PR (right), same image:

Checklist

• I have read and confirmed this PR follows the contribution guidelines.

[leejet]

Superseded by #2045 .