feat(neuron): honour HF_HUB_CACHE / HF_HOME for the candle harness cache

Resolves the candle harness's HuggingFace cache directory with the
following precedence (first hit wins):

1. Explicit `hf_cache` in `[harness.candle]` from neuron.toml.
2. `HF_HUB_CACHE` env var — the Python `huggingface_hub` convention.
   The Rust hf-hub crate doesn't read this natively, so we bridge here.
3. `HF_HOME` env var (`$HF_HOME/hub` per the canonical layout).
4. None — falls through to hf-hub's own default.

Honouring HF_HUB_CACHE lets a neuron host reuse an existing cache
directory shared with Python tooling or other harnesses on the same
host without per-tool config. The canonical per-host setup is a
systemd drop-in:

    /etc/systemd/system/neuron.service.d/local.conf
    [Service]
    Environment=HF_HUB_CACHE=/archive/hf-cache

neuron.example.toml documents the resolution chain inline.

script/validate-neuron.sh: bump LOAD_TIMEOUT from 600s to 3600s and
expose both load/infer timeouts via env (NEURON_LOAD_TIMEOUT,
NEURON_INFER_TIMEOUT). A Qwen3.6-class dense model is ~54 GB and was
hitting the 10-min ceiling cold-downloading on a residential link.

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

neuron.example.toml

@@ -19,8 +19,21 @@ name = "candle"
 # Optional tuning for the candle harness.
 
 [harness.candle]
-# HuggingFace cache directory for model weights. When unset, hf-hub's
-# default (~/.cache/huggingface) is used.
+# HuggingFace cache directory for model weights.
+#
+# Resolution order (first hit wins):
+#   1. `hf_cache` here in this file.
+#   2. `HF_HUB_CACHE` env var — same convention as the Python
+#      `huggingface_hub` library, so an existing cache directory shared
+#      with other tooling can be reused without per-tool config.
+#   3. `HF_HOME` env var (cache appended as `$HF_HOME/hub`).
+#   4. hf-hub's default (`~/.cache/huggingface/hub`).
+#
+# For per-host overrides (e.g. one neuron has an SSD with prefetched
+# weights), prefer a systemd drop-in over editing this file:
+#   /etc/systemd/system/neuron.service.d/local.conf:
+#     [Service]
+#     Environment=HF_HUB_CACHE=/archive/hf-cache
 # hf_cache = "/var/lib/neuron/hf-cache"
 
 # -- Default models ----------------------------------------------------------