common : auto-download dflash- and eagle3- HF sidecars (#25811)

* common: auto-download dflash- and eagle3- HF sidecars

Mirror the existing mtp- sidecar logic to support auto-discovery and
download of DFlash (dflash-) and Eagle3 (eagle3-) speculative decoding
sidecars from Hugging Face repos.

Changes:
- Add --dflash and --eagle3 CLI flags to trigger sidecar download
- Add find_best_dflash() and find_best_eagle3() using find_best_sibling
- Exclude dflash- and eagle3- filenames from primary model selection
- Filter dflash- and eagle3- from cached model listings
- Wire download tasks that set speculative.draft.mparams as fallback

Assisted-by: pi:llama.cpp/Qwen3.6-27B

* docs : regen

common/download.h

@@ -55,8 +55,10 @@ struct common_download_opts {
     std::string bearer_token;
     common_header_list headers;
     bool offline = false;
-    bool download_mmproj = false;
-    bool download_mtp = false;
+    bool download_mmproj  = false;
+    bool download_mtp     = false;
+    bool download_eagle3  = false;
+    bool download_dflash  = false;
     common_download_callback * callback = nullptr;
 };
 
@@ -106,6 +108,8 @@ struct common_download_hf_plan {
     hf_cache::hf_files model_files;
     hf_cache::hf_file mmproj;
     hf_cache::hf_file mtp;
+    hf_cache::hf_file eagle3;
+    hf_cache::hf_file dflash;
     hf_cache::hf_file preset; // if set, only this file is downloaded
 };
 common_download_hf_plan common_download_get_hf_plan(const common_params_model & model, const common_download_opts & opts);