[Validation/Docs] Add release checks and docs for shell/profile boundaries

[ogyrec-o]

Summary

Add release/package validation and documentation that prevents shell/profile boundaries from regressing.

This issue is the guardrail pass after the product-shell packaging split.

Problem

Once Freven supports multiple artifact/product profiles, accidental regressions become likely:

• host-only artifact accidentally includes Vanilla
• standalone template accidentally depends on Vanilla
• Vanilla product forgets required Vanilla payload
• shell-specific defaults leak into generic host foundation
• docs tell users to use the wrong artifact

We need checks and docs that make these boundaries visible and enforceable.

Requirements

Add validation/checks for:

• host-only artifact contains no freven.vanilla payload
• host-only artifact does not expose Vanilla builtin registry unless explicitly configured
• Vanilla product artifact includes required Vanilla experience/content
• standalone custom-game template contains no Vanilla dependency by default
• DevKit profile manifests identify their profile kind
• packaging docs match actual artifact contents
• generic host foundation docs do not describe Vanilla as universal truth

Docs should explain:

• which artifact/profile to download as a player
• which artifact/profile to download as a mod author
• which artifact/profile to use for standalone game development
• how Vanilla differs from the generic host
• how experience.stack.toml fits into host/Vanilla/modpack workflows

Non-goals

• Do not create new shell functionality.
• Do not redesign packaging again.
• Do not implement online catalog/search.
• Do not solve unrelated SDK/rendering/worldgen features.

Deliverables

• Package inspection checks or CI scripts.
• Updated release checklist.
• Updated DevKit docs.
• Updated product-shell docs.
• Clear pass/fail examples for each profile.