model: support speculators-format checkpoints for DSpark (#26275)

* dspark: support speculators-format checkpoints (SpecForge exports)

Speculators-format DSpark drafts (e.g. SpecForge exports for the
Gemma-4-26B-A4B target) differ from the dense DeepSpec checkpoints in
three ways:

- the config nests the backbone hparams under transformer_layer_config
  and gives the extract layers as aux_hidden_state_layer_ids
- the block is the DFlash 1+N fill-in layout: the anchor slot is a bonus
  token, not a prediction slot. Written as dflash.bonus_anchor; such
  drafts build the block and read the mask positions exactly like
  DFlash (n_max drafts from a 1+n_max block), only the Markov/confidence
  sampling comes from DSpark
- the draft output vocab may be reduced (draft_vocab_size < vocab_size)
  with a d2t remap table. The converter expands lm_head/markov_w2 back
  to the full vocab and synthesizes an lm_head bias of -1e9 on the rows
  the draft cannot produce, so the runtime needs no d2t remapping. Such
  drafts ship their own (now optional) token_embd/output tensors instead
  of sharing the target's

Verified against gemma4-26b-a4b-dspark: greedy outputs are byte-identical
with and without the draft; acceptance 0.46, mean draft len 3.7 (n_max 6).

Co-authored-by: desovo7 <942845546@qq.com>
Assisted-by: Claude Fable 5

* dspark: fold the speculators draft class into DSparkModel

One class now covers every DSpark variant. What used to pick the class is
a single flag, because the arch name turns out to be the only thing that
separates the two families: SpecForge also exports a flat schema that
carries no speculators_* fields yet still uses the 1+N bonus-anchor block,
so keying on those fields would silently mis-read its drafts.

Also rename i0 to i_first_pred in the draft read loop and the Markov head,
and give the head a real bonus_anchor bool instead of testing i0 > 0.

Converting the Qwen3-8B DeepSpec draft and both gemma-4 speculators drafts
produces byte-identical GGUFs. The one behaviour change is that the
markov_head_type check now also covers the DeepSpec checkpoints, which
previously skipped it.

Co-authored-by: desovo7 <942845546@qq.com>
Assisted-by: Claude Opus 5

* dspark: address review comments

- rename bonus_anchor to sample_from_anchor (GGUF key and code), matching
  the checkpoint config field; absent key still means anchor-first
- rework the reduced draft vocab to match EAGLE3: d2t is written as I64
  absolute target ids and the logits are scattered at runtime, instead of
  expanding lm_head/markov_w2 and synthesizing an output bias at conversion
- move the t2d skip to modify_tensors, like EAGLE3
- drop _is_specforge: the arch name only picks the sample_from_anchor
  default, embed/lm_head sharing is decided by the draft vocab size
- deduplicate the tok_embd create_tensor left behind by the rebase

Verified with the RedHat gemma-4-31b speculator draft: greedy output is
byte-identical with and without the draft; acceptance 0.26 (n_max 7).

Co-authored-by: desovo7 <942845546@qq.com>
Assisted-by: Claude Fable 5

* dspark: fold the sample_from_anchor read into the block_size block

* dspark: fix flake8 continuation indent

* clean up

* dspark: key the sample_from_anchor default off the export format

  Co-authored-by: desovo7 <942845546@qq.com>
  Assisted-by: Claude Fable

* dspark: drop t2d in filter_tensors

  Co-authored-by: desovo7 <942845546@qq.com>
  Assisted-by: Claude Fable

* dspark: map model.lm_head instead of bypassing the dflash prefix

  Co-authored-by: desovo7 <942845546@qq.com>
  Assisted-by: Claude Fable

---------

Co-authored-by: desovo7 <942845546@qq.com>
Co-authored-by: ruixiang63 <wangruixiang07@outlook.com>

common/speculative.cpp

@@ -926,6 +926,9 @@ struct common_speculative_impl_draft_dflash : public common_speculative_impl {
     // draft-dspark: the draft carries a Markov head and uses an anchor-first block layout
     const bool is_dspark;
 
+    // dspark speculators
+    bool sample_from_anchor = true;
+
     const int32_t * target_layer_ids   = nullptr; // model_dft's extract layer indices
     uint32_t        target_layer_ids_n = 0;
 
@@ -960,16 +963,20 @@ struct common_speculative_impl_draft_dflash : public common_speculative_impl {
             if (llama_model_meta_val_str(model_dft, "dflash.block_size", buf, sizeof(buf)) >= 0) {
                 block_size = std::atoi(buf);
             }
+            if (llama_model_meta_val_str(model_dft, "dflash.sample_from_anchor", buf, sizeof(buf)) >= 0) {
+                sample_from_anchor = std::strcmp(buf, "true") == 0;
+            }
         }
         mask_token_id = llama_vocab_mask(llama_model_get_vocab(model_dft));
 
         LOG_INF("%s: adding speculative implementation '%s'\n", __func__, common_speculative_type_to_str(type).c_str());
         LOG_INF("%s: - n_max=%d, n_min=%d, p_min=%.2f\n", __func__, this->params.n_max, this->params.n_min, this->params.p_min);
-        LOG_INF("%s: - block_size=%d, mask_token_id=%d, n_extract=%u\n", __func__, block_size, mask_token_id, target_layer_ids_n);
+        LOG_INF("%s: - block_size=%d, mask_token_id=%d, n_extract=%u, sample_from_anchor=%s\n", __func__,
+                block_size, mask_token_id, target_layer_ids_n, sample_from_anchor ? "true" : "false");
 
         // DFlash input is [id_last, <mask> * (block_size-1)]: in-place denoising yields at most
-        // block_size-1 draft tokens, DSpark yield a full block_size draft tokens
-        const int32_t n_draft_max = is_dspark ? block_size : block_size - 1;
+        // block_size-1 draft tokens, anchor-first DSpark yields a full block_size draft tokens
+        const int32_t n_draft_max = is_dspark && sample_from_anchor ? block_size : block_size - 1;
         if (this->params.n_max > n_draft_max || this->params.n_min > n_draft_max) {
             LOG_WRN("%s: requested draft size (n_max=%d, n_min=%d) exceeds the trained block size %d -- clamping to %d\n",
                     __func__, this->params.n_max, this->params.n_min, block_size, n_draft_max);
@@ -1175,7 +1182,7 @@ struct common_speculative_impl_draft_dflash : public common_speculative_impl {
 
             const int32_t n_draft = params.n_max;
 
-            const int32_t n_block_tokens = n_draft + (is_dspark ? 0 : 1);
+            const int32_t n_block_tokens = n_draft + (is_dspark && sample_from_anchor ? 0 : 1);
             i_block_beg[seq_id] = batch.n_tokens;
             n_block    [seq_id] = n_block_tokens;
             for (int32_t i = 0; i < n_block_tokens; ++i) {
@@ -1208,11 +1215,11 @@ struct common_speculative_impl_draft_dflash : public common_speculative_impl {
             auto & result = *dp.result;
 
             if (is_dspark) {
-                // DSpark predicts the next token from position 0 and optionally truncates
-                // at the first position below the confidence threshold.
+                // DSpark: read from the first draft slot, truncate below the confidence threshold
                 const float * conf = params.p_min > 0.0f ? llama_get_embeddings_nextn(ctx_dft) : nullptr;
-
-                for (int32_t i = 0; i < n_block_tokens; ++i) {
+                // bonus-anchor drafts read the mask positions only, like DFlash
+                const int32_t i_draft_beg = sample_from_anchor ? 0 : 1;
+                for (int32_t i = i_draft_beg; i < n_block_tokens; ++i) {
                     const int32_t idx = beg + i;
 
                     if (conf && conf[(size_t) idx * n_embd_dec] < params.p_min) {