Document component gallery acceptance

[jmchilton]

What

• add component-gallery acceptance guidance to the standing-up-a-foundry instructions
• make the shared SPECIMENS registry the required source for site-kit examples
• define local specimen groups, standalone routes, theme ownership, built-output assertions, inbound linking, search policy, and style-gap checks
• point adopters to the live foundry-lib reference gallery documentation

Why

The pattern already described site-kit ownership, but it did not explain how a foundry should prove that shared components work under its own visual system or how domain-specific flourishes should live beside shared examples. This fills that gap while keeping reusable gallery machinery upstream and foundry-specific presentation local.

Impact

Instructional guidance only; existing foundries and runtime behavior are unchanged.

Checks

• test suite (22 passing)
• Astro typecheck (0 errors)
• wiki-link check
• production build (28 pages)