feat(neuron): load default_models on service activation

Stage 5 of the candle-native pivot. Adds first-class support for
auto-loading a configured set of models when the neuron service
activates.

Config:
- NeuronConfig.default_models: Vec<ModelSpec> (defaults to []).
- neuron.example.toml ships a commented [[default_models]] example.

Activation flow (crates/neuron/src/startup.rs::load_default_models):
- Sequential — VRAM contention makes parallel loads risky.
- Per-entry timing logged at info level on success.
- Failures logged as warnings; the next entry is still attempted.
- An empty list short-circuits without log noise.

Called from main.rs after the registry is built and before the axum
listener binds, so /models reflects the loaded state from the very
first request.

data/neuron.service gains TimeoutStartSec=1800s. With activation
blocked on potentially slow first-time HF downloads + GGUF
materialisation, systemd's default 90s would kill larger model loads
mid-flight.

Two non-gated tests in tests/activation.rs cover the
continues-past-failure and empty-list paths using a synthetically
unknown harness name to fail loads fast without touching the network.
The cuda-integration test from earlier stages still exercises the
real load/unload lifecycle.

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

neuron.example.toml

@@ -22,3 +22,19 @@ name = "candle"
 # HuggingFace cache directory for model weights. When unset, hf-hub's
 # default (~/.cache/huggingface) is used.
 # hf_cache = "/var/lib/neuron/hf-cache"
+
+# -- Default models ----------------------------------------------------------
+# Models listed here are loaded automatically when the neuron service
+# activates. Loading is sequential — a slow or failing entry doesn't
+# block the rest of the fleet, but it does push out the time before
+# neuron starts serving HTTP, so keep the list short. Operators can
+# load additional models on demand via POST /models/load.
+#
+# Make sure data/neuron.service's TimeoutStartSec is generous enough to
+# cover the slowest entry's first-time download + materialisation.
+
+# [[default_models]]
+# model_id = "Qwen/Qwen3-0.6B-GGUF"
+# harness = "candle"
+# quant = "Q4_K_M"
+# devices = [0]