docs: humanize README and add Realm preview

[ael-dev3]

Summary

• rewrite the root README around a plain-language hook, four-sentence introduction, minimal quick start, compact status table, five-word stack reasons, and focused links
• preserve every existing README link and the project’s factual, security, financial, authority, and licensing boundaries
• move the explicit runtime-asset contributor check into CONTRIBUTING.md
• add the owner-supplied Realm screenshot with exact provenance and an explicit development-preview caption because the procedural grass layer from Visual: add procedural biome grass and wind #55 is not live in Alpha 0.3.8

Truth and scope

This is documentation-only. It is based on protected main at Alpha 0.3.8 and does not change runtime code, authentication, SpacetimeDB, production data, deployment state, or any other pull request. The screenshot remains LicenseRef-Warpkeep-Provenance-Required and is not a runtime dependency.

Validation

• focused README, community-intake, and license-policy tests: 32 passed across 3 files before the screenshot addition; 7 passed across 2 README/community files after it
• license-policy verification passed
• tracked file-size policy passed
• all README local link and image targets exist
• all original README links remain present
• README is 64 lines; required section order and wording checks passed
• git diff --check passed