convert : write explicit recurrent_layers for Qwen3-Next / Qwen3.5 (#28208)

Problem
- Loader prefers `<arch>.attention.recurrent_layers`, falls back to `full_attention_interval` if missing
- Converter only ever writes the interval. gguf-py has no constant/writer for the array
- Interval can only describe evenly spaced full-attention layers. Any non-uniform `layer_types` gets reconstructed wrong
- No error, no warning. Model loads, runs, wrong layers get wrong ops. Full-attn layers marked recurrent lose their KV cache
- Every published Qwen3.5 checkpoint is uniform so nobody's hit it yet

Repro
12 layers, periods 4/3/5:

    layer:  0 1 2 3 4 5 6 7 8 9 10 11
    actual: L L L F L L F L L L  L  F
    loader: L L L F L L L F L L  L  F
                        ^ ^

Layer 6 is full attn, loaded as recurrent. Layer 7 the reverse.
52-layer non-uniform stack: 15/52 mis-typed.

Fix
- `constants.py`: add `Keys.Attention.RECURRENT_LAYERS` (name already registered in llama-arch.cpp)
- `gguf_writer.py`: add `add_recurrent_layers()`, same shape as `add_rope_pattern()`
- `conversion/qwen.py`: emit array from `layer_types` in `Qwen3NextModel.set_gguf_parameters` (covers 3-Next, 3.5, 3.5-MoE)

Notes
- Array is padded with `false` for MTP blocks. `get_key_or_arr` checks length against `n_layer_all`, which includes MTP. Matches the fallback's `i < n_layer()` guard
- Interval is still written. Old builds only understand the interval
- `layer_types` length != `num_hidden_layers` now raises in converter instead of producing a GGUF that fails at load

Tested
- End-to-end on a 62-layer non-uniform Qwen3.8-27B (2 linear layers removed). Loader reads the array, 62 blocks, 0 mismatches. Without fix: interval fallback, mis-typed
- MTP padding NOT tested on a real MTP model. Reasoned from qwen35.cpp + get_key_or_arr. Would appreciate a check

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

conversion/qwen.py

@@ -379,6 +379,13 @@ class Qwen3NextModel(_QwenMtpMixin, Qwen2MoeModel):
         self.gguf_writer.add_ssm_group_count(self.hparams["linear_num_key_heads"])
         self.gguf_writer.add_ssm_time_step_rank(self.hparams["linear_num_value_heads"])
         self.gguf_writer.add_ssm_inner_size(self.hparams["linear_value_head_dim"] * self.hparams["linear_num_value_heads"])
+        if (layer_types := self.hparams.get("layer_types")) is not None:
+            n_layer = self.hparams["num_hidden_layers"]
+            if len(layer_types) != n_layer:
+                raise ValueError(f"layer_types has {len(layer_types)} entries, expected num_hidden_layers ({n_layer})")
+            recurrent = [t == "linear_attention" for t in layer_types]
+            recurrent += [False] * (self.block_count - n_layer)
+            self.gguf_writer.add_recurrent_layers(recurrent)
         self.gguf_writer.add_full_attention_interval(self.hparams.get("full_attention_interval", 4))
         if (rope_dim := self.hparams.get("head_dim")) is None:
             rope_dim = self.hparams["hidden_size"] // self.hparams["num_attention_heads"]

gguf-py/gguf/constants.py

@@ -215,6 +215,7 @@ class Keys:
         KV_LORA_RANK_SWA             = "{arch}.attention.kv_lora_rank_swa"
         SHARED_KV_LAYERS             = "{arch}.attention.shared_kv_layers"
         SLIDING_WINDOW_PATTERN       = "{arch}.attention.sliding_window_pattern"
+        RECURRENT_LAYERS             = "{arch}.attention.recurrent_layers"
         TEMPERATURE_SCALE            = "{arch}.attention.temperature_scale"
         ROPE_PATTERN                 = "{arch}.attention.rope_pattern"
 

gguf-py/gguf/gguf_writer.py

@@ -841,6 +841,9 @@ class GGUFWriter:
         else:
             self.add_array(key, value)
 
+    def add_recurrent_layers(self, value: Sequence[bool]) -> None:
+        self.add_array(Keys.Attention.RECURRENT_LAYERS.format(arch=self.arch), value)
+
     def add_rope_pattern(self, value: Sequence[bool]) -> None:
         self.add_array(Keys.Attention.ROPE_PATTERN.format(arch=self.arch), value)