Adds DFlash speculative decoding as a per-arch model class:

* src/models/dflash.cpp (new): `llama_model_dflash` — block-diffusion
  drafter that proposes N tokens per step against the target model's
  context-conditional embeddings. SWA-aware attention mask, n_block
  noise tokens layered against ctx tokens, per-layer is_swa_impl
  routing.
* src/llama-arch.{cpp,h}: LLM_ARCH_DFLASH registered (NeoX rope).
* src/llama-graph.{cpp,h}: `llm_graph_input_dflash` carries target
  hidden + masks, sets them on the graph each ubatch.
* src/llama-model{.cpp,.h,-loader.cpp}: arch dispatch, tensor loader
  hooks, std::array<int,5> instantiation for the noise schedule.
* src/llama-cparams.h + llama-hparams.h + llama-context.h +
  llama.cpp: cparams flag + ctx wiring + arch-dispatch entry.
* src/models/{gemma4,llama,models}.* : hooks so the existing target
  archs cooperate with the dflash drafter.
* common/speculative.cpp + speculative.h: COMMON_SPECULATIVE_TYPE_DFLASH
  config; auto-enable logic that gates 'draft' type on actual draft-
  model path presence while leaving room for dflash / eagle3 / mtp.
* tools/server: router-aware integration (last_used_ms field,
  pick_any_resident wired with dflash flag), server-task.h/cpp + queue
  + server.cpp glue, --parallel 1 gate left in place per current
  Round-11 status.
* tests/test-dflash.cpp + scripts/* + HANDOFF.md: smoke harness,
  bench, weight-compare, regression artifacts.

Squashed from 42 commits on the feat/dflash-integration branch (the
previous Round-11 lifecycle iterations). Rebased onto the post-rewrite
ht baseline; conflicts resolved against the per-arch class hierarchy
that's now upstream-stock (renamed swa_layers → is_swa_impl, single
remap_developer_role definition in server_chat_params, has_draft_simple
auto-enable kept dflash-aware).