Refine stability definitions

[captainbrosset]

This is part of the WebDX CG's 2026 roadmap.

• Shift tracking from "Spec Stability" to "Feature Stability" to accurately reflect the usability of specific capabilities (e.g., similar to Media Capabilities) rather than entire documents.
• Link tests more naturally with specification updates to catch interoperability bugs (like anchor-positioning issues) earlier in the standardization process.