feat: DDTree draft + hybrid tree-attention verify for DFlash

Port Lucebox DDTree (top-K extract, best-first tree build, follow) into
common/, wire draft-dflash for chain and tree drafts, and add GDN/SSM
TREE_MODE CUDA kernels plus ancestor KQ mask. Server path packs a DFS
batch with parent_ids/visibility, walks target logits, then checkpoint-
restores and chain-commits the accepted path.

Enable with LLAMA_DFLASH_DDTREE=1 and LLAMA_DFLASH_TREE_VERIFY=1. Smoke
shows higher mean accept length than chain verify, with higher verify
cost until intermediate-state commit is available.