fix(neuron/candle): source tokenizer.json from base repo when GGUF

GGUF-only HF repos (unsloth/Qwen3-*-GGUF, Qwen/Qwen3-*-GGUF) ship the
.gguf file but not tokenizer.json — the tokenizer data is embedded in
the GGUF metadata itself, and the standalone tokenizer.json lives in
the base non-GGUF repo (unsloth/Qwen3-0.6B, Qwen/Qwen3-0.6B, etc.).

Live validation against quadbrat hit:
  HTTP 400 fetch tokenizer.json from unsloth/Qwen3-0.6B-GGUF:
  HTTP status client error (404 Not Found)

resolve_files now derives the tokenizer repo by stripping a `-GGUF`
or `-gguf` suffix from the model_id; non-GGUF ids fall through to
fetching from the same repo. The error message includes the
attempted tokenizer repo id so the next failure (e.g. base repo
doesn't exist) is unambiguous.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/harness/candle.rs

@@ -121,10 +121,33 @@ impl CandleHarness {
             .get(&gguf_filename)
             .await
             .with_context(|| format!("fetch GGUF {gguf_filename}"))?;
-        let tokenizer_path = repo
+
+        // GGUF-only HF repos (unsloth/Qwen3-*-GGUF, Qwen/Qwen3-*-GGUF,
+        // etc.) ship the .gguf file but not tokenizer.json — the
+        // tokenizer.json lives in the base non-GGUF repo. Derive the
+        // base repo id by stripping a `-GGUF` / `-gguf` suffix; if
+        // there's no such suffix the same repo is used (works for
+        // non-GGUF model_ids).
+        let tokenizer_repo_id = spec
+            .model_id
+            .strip_suffix("-GGUF")
+            .or_else(|| spec.model_id.strip_suffix("-gguf"))
+            .unwrap_or(spec.model_id.as_str())
+            .to_string();
+        let tokenizer_repo = if tokenizer_repo_id == spec.model_id {
+            repo
+        } else {
+            tracing::debug!(
+                from = %spec.model_id,
+                to = %tokenizer_repo_id,
+                "tokenizer.json sourced from base repo (GGUF suffix stripped)"
+            );
+            api.model(tokenizer_repo_id.clone())
+        };
+        let tokenizer_path = tokenizer_repo
             .get("tokenizer.json")
             .await
-            .context("fetch tokenizer.json")?;
+            .with_context(|| format!("fetch tokenizer.json from {tokenizer_repo_id}"))?;
         Ok((gguf_path, tokenizer_path))
     }