Generate manifest-backed assets in Amber V2 web apps

[crimson-knight]

What changed

• Adds amber assets build and amber assets check.
• Generates authored files under app/assets and compiled output under ignored public/assets.
• Builds the first manifest during project generation and before watch recompiles.
• Updates the default layout, import map, starter styling, and Amber crystal artwork to use asset helpers.
• Expands Unix and Windows smoke tests to boot the generated app and request real fingerprinted assets.
• Keeps the default Grant, Micrate, and SQLite persistence path introduced in the preceding V2 template PR.

Why

A fresh Amber V2 app should have a production-safe frontend and database path immediately, without asking a new user to invent either one.

Validation

• CLI suite: 410 examples, 0 failures
• Beta contract checker
• Generated-app smoke: asset build/verify, SRI, MIME, caching, gzip, migrations, specs, release build, and Pet CRUD