Architecture Explorer: expand model support beyond the curated list

## Context

Feature 004 removed the Architecture Explorer's free-text "any open-weights HuggingFace id" input and replaced it with a curated dropdown (FR-412). That was the honest move: the static build's live path depends on a community **ONNX** export existing for the model, and most HF repos have none — so the old input promised something the deployed demo could not deliver.

This issue tracks doing it properly.

## Options to evaluate

1. **Grow the curated list.** Cheap and safe. Each entry needs a verified ONNX export (`onnx-community/...`) plus a pinned revision, and needs to fit under the parameter ceiling (`ARCH_MAX_PARAMS`).
2. **Automatic filtering.** Query the HF API for models that (a) carry an open-weights license, (b) have an ONNX export with the file layout transformers.js expects, and (c) are under the size gate — then offer *those* as a searchable list. This keeps the promise honest because the filter is the promise.

## Constraints worth remembering

- ONNX exports do **not** expose attention weights or hidden states, so live per-prompt *tracing* of an arbitrary model in the browser is not possible even when generation works. Any expansion must be explicit about which capabilities a given model gets (generate / tokenize / weight-inspect / trace).
- The full-stack (Python) path can trace anything HF can load; only the static build is constrained. Copy must not blur the two.

## Acceptance

- The picker's affordance matches exactly what each listed model can do.
- No model appears that the runtime cannot actually load.
- Red-team pass finds no claim contradicted by behavior.