feat(cortex): catalogue source field + scheme-qualified /models/load

Phase 3 of plan-source-aware-loader-preflight. Adds an optional
`source` field to `ModelProfile` and threads it through the
router's cold-load path so a profile pointing at the helexa
registry forwards `helexa:<id>` to neuron's `/models/load`
instead of leaving neuron to substitute its `default_source`
(typically `huggingface`).

Without this, an operator who declares
`source = "helexa"` in models.toml would still see neuron fetch
from HuggingFace — the catalogue → ModelSpec translation in
`profile_to_spec` was dropping the scheme on the floor.

What lands:

- `cortex-core::catalogue::ModelProfile.source: Option<String>`.
  None is the default and preserves pre-Phase-3 behaviour.
- `cortex-gateway::router::qualified_model_id(profile)` —
  small pure helper, extracted from `profile_to_spec` so it can
  be unit-tested. Empty-string `source` is treated as None so
  operators who blank out a previously-set value don't trip a
  scheme-with-no-scheme failure mode in neuron.
- `models.example.toml` documents the new field with a
  commented-out helexa-scheme example pointing back at
  neuron.example.toml's matching sources block.

Tests:

- 2 new unit tests in `cortex-core::catalogue`: source-absent
  round-trip and source-present round-trip through TOML.
- 3 new unit tests in `cortex-gateway::router`: pass-through
  when None, prefix when Some, pass-through on empty-string
  source.
- ModelProfile literal in catalogue's existing test updated to
  carry `source: None`.

CI gate: cargo fmt --check, cargo clippy --workspace
--all-targets -- -D warnings, cargo test --workspace
(24 test groups ok, zero failures).

Completes Phase 3. With Phases 1+2+3 landed:
- neuron parses `scheme:org/name`, routes per-source hf-hub
  Api with disambiguated cache.
- preflight returns structured errors before any device
  allocation.
- cortex catalogue declares per-model source jurisdiction
  and forwards it to neuron.

The registry itself (registry.helexa.ai service, MinIO,
nginx, mirror fabric) is the next moving piece — landing
under a separate project per the design discussion.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

models.example.toml

@@ -7,7 +7,8 @@
 # returns and what the router can cold-load on demand.
 #
 # Field reference:
-#   id                 - HuggingFace model id, exact match.
+#   id                 - Repo id in the source registry (e.g. "Qwen/Qwen3.6-27B").
+#                        Exact match.
 #   harness            - which engine handles inference (currently "candle").
 #   quant              - GGUF quantisation tag for the file in the HF repo
 #                        (e.g. "Q4_K_M"). Omit/empty for the dense
@@ -20,6 +21,11 @@
 #   pinned_on          - optional whitelist of neuron names. Non-empty
 #                        narrows feasibility to just those neurons and
 #                        protects the model from LRU eviction there.
+#   source             - optional source scheme ("huggingface", "helexa",
+#                        operator mirror tag). When set, cortex forwards
+#                        the load to neuron as `scheme:id` so the daemon
+#                        fetches from the right registry. Omit to let
+#                        neuron substitute its own `default_source`.
 
 # Tensor-parallel target — needs a neuron with at least 2 large GPUs.
 # The example pins to a specific neuron name; adjust or remove the
@@ -49,6 +55,20 @@ vram_mb = 500
 min_devices = 1
 min_device_vram_mb = 4000
 
+# Helexa registry model — `source` pins this entry to the helexa
+# scheme so cortex forwards `helexa:Helexa/Qwen3.6-27B-Uncensored` to
+# neuron's /models/load. Requires the neuron config to declare a
+# matching [harness.candle.sources.helexa] entry pointing at the
+# helexa registry endpoint (see neuron.example.toml).
+#
+# [[models]]
+# id = "Helexa/Qwen3.6-27B-Uncensored"
+# harness = "candle"
+# source = "helexa"
+# vram_mb = 54000
+# min_devices = 2
+# min_device_vram_mb = 24000
+
 # -- Tier aliases ------------------------------------------------------------
 # Optional. Clients can request inference against an alias (e.g.
 # `model: "helexa/small"` in /v1/chat/completions) and cortex