Fix causal attention in candle-flash-attn-v3: NULL tile_count_semaphore crash (dense) + silently non-causal varlen#3606

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Xueying-VirtueAI:fa3-fix-causal

Commits on Jun 9, 2026

Fix 128x softmax_lse over-allocation in dense flash-attn-v3 path

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Commits on Jun 11, 2026

Fix illegal NULL atomic in causal flash-attn-v3 (allocate tile_count_semaphore)

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Fix silently non-causal flash_attn_varlen (window-size clamp clobbered the causal flag)

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