Template Grading Rubric: image/CSS exceptions can't score A, and Image Handling cites the wrong CDN

## Summary

The **Template Grading Rubric** in the [Contributing-Templates wiki](https://github.com/facebookexperimental/xds/wiki/Contributing-Templates#template-grading-rubric) has two internal contradictions that cause it to penalize templates for following its own documented rules. Surfaced while grading and improving the `classic-gallery` page template.

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## Issue 1 — "Necessary exceptions" are still capped below A (the ceiling bug)

The rubric explicitly lists `` (with an `xds_oss` CDN src) and `` wrapping `XDSFormLayout` as **necessary exceptions** that should **not** be penalized:

> **Necessary exceptions** (don't penalize):
> - `` with xds_oss CDN src (XDS doesn't have a general Image component)
> - `` wrapping `XDSFormLayout` for native submission semantics

But the scoring **tables** only award full marks at *literally zero* raw elements / declarations:

**XDS Component Purity (30 pts)**
| Raw HTML elements | Points |
|---|---|
| 0 | 30 |
| 1–2 (all necessary) | 25 |

**Custom CSS (15 pts)**
| Total custom style declarations | Points |
|---|---|
| 0 | 15 |
| 1–3 (justified — no XDS alternative) | 12 |

### Why this is a contradiction

Any **image gallery** needs at least one ``. A cover-fit image also irreducibly needs `width: 100%`, `height: 100%`, `object-fit: cover` (3 declarations) plus the `stylex.props()` call site — none of which have an XDS prop equivalent (there is no XDS `Image` component).

Result: a *perfectly built* image template is hard-capped at roughly:
- Purity **25/30** (one necessary ``)
- Custom CSS **5/15** (cover-fit lands in the "4–10" band)

…i.e. **~85 (B) maximum, and an A (90+) is impossible**, no matter how clean the code is. The rubric says "don't penalize" the exception, but the math penalizes it ~15 points and blocks the top grade.

This affects a whole class of templates (galleries, hero images, anything media-heavy). Notably the **official `AspectRatioImageGallery` block** (`packages/cli/templates/blocks/components/AspectRatio/AspectRatioImageGallery.tsx`) uses `` with inline `style={{ objectFit: 'cover', ... }}` — it would also be capped by its own rubric.

### Suggested fix
- Make the "necessary exception" actually neutral in the tables, e.g.:
- **Purity:** "0 raw HTML *(excluding necessary ``/`` exceptions)* → 30."
- **Custom CSS:** exclude the irreducible image cover-fit declarations (`width/height/object-fit` on an `` inside `XDSAspectRatio`) from the count, or add an "image cover-fit only" full-credit row.
- Alternatively, cap the *deduction* for documented exceptions at 0 so a clean gallery can reach A.

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## Issue 2 — Image Handling cites `scontent…fbcdn.net`, contradicting the wiki's own image guide

**Category 6 — Image Handling (5 pts)** says:

> - **Correct**: CDN URLs from `xds_oss` asset set (`https://scontent.xx.fbcdn.net/...`).

But the same wiki's **"How to Use Images in Templates"** section says the opposite — use the **permanent `lookaside.facebook.com`** URLs and explicitly **warns against** the `scontent…fbcdn.net` signed URLs:

> Use the **permanent lookaside URL** — it never expires and requires no authentication:
> `https://lookaside.facebook.com/assets/{set}/{asset}.{ext}`
>
> > ⚠️ **Do NOT use the signed CDN URLs** (the `https://scontent.xx.fbcdn.net/...?oh=...&oe=...` URLs). Those contain time-limited tokens that expire after a few days.

So the rubric grades "correct" using the exact URL form the guide says is wrong (and which expires). A template using the recommended `lookaside.facebook.com` URLs technically doesn't match the rubric's stated "correct" pattern.

### Suggested fix
Update Category 6 to cite `https://lookaside.facebook.com/assets/xds_oss/...` as the correct form and list expiring `scontent…fbcdn.net` signed URLs under **Incorrect**, matching the image guide.

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## Repro / context

Found while grading `classic-gallery` (`packages/cli/templates/pages/classic-gallery/page.tsx`). After refactoring it to be genuinely clean (raw `