Bug: `parseCategory` drops unknown YAML fields (e.g. `datasets:`) via rest spread, dropping MIMIC-IV Demo from build

[mmcdermott]

Summary

`parseCategory` destructures only `name, description, icon, packages` and shoves every other key into a "subcategories" rest spread. Any non-recognized field whose value isn't another category-shaped object gets logged as `Invalid subcategory format` and silently dropped.

Repro

`npm run build` currently emits:
```
Invalid subcategory format for physionet_repo: https://physionet.org/content/mimic-iv-demo-meds/0.0.1/
```

Code pointers

• `src/lib/ecosystem/utils.ts` line 58 — destructure that creates the rest-spread of "subcategories":
  ```ts
  const { name, description, icon, packages: rawPackages, ...rawSubcategories } = rawCategory;
  ```
• `src/lib/ecosystem/utils.ts` line 124 — the warn:
  ```ts
  console.warn(`Invalid subcategory format for ${key}:`, rawSubcategory);
  ```
• `static/data/ecosystem.yaml` lines 81–87 — the trigger:
  ```yaml
  publicly_available_datasets:
  name: 'Public Datasets'
  description: '...'
  datasets: # <-- not in the destructure; treated as a subcategory map
  - name: 'MIMIC-IV Demo'
  physionet_repo: https://physionet.org/content/mimic-iv-demo-meds/0.0.1/
  ```

Symptom

`MIMIC-IV Demo` (and any future `datasets:` entries) never reach the rendered ecosystem page. The build logs the warning but nothing surfaces it as a failure, so this regression can land silently.

Suggested fix

Two layers:

1. Parser: explicitly recognize `datasets:` (and any other intentional non-package keys), or replace the rest-spread with an explicit `subcategories:` field, so unknown keys are caught at parse time rather than treated as silent subcategory maps.
2. CI: consider failing the build on these warnings (or surfacing them as console errors) so future drift is caught.

Found while doing a repo health pass; reproduced via `npm run build` on `main`.