docs(spec): require outermost DOM styling target - #6066

Summary

Proposes one rule: when a stable Core component accepts style, className, or xstyle, every accepted prop applies to the same outermost DOM element. A component without one stable root must not expose unsupported general styling props.

Why

Today Astryx requires accepted styling props to be forwarded and composed, but does not define their DOM target. Consumers may need implementation knowledge to know whether they style a wrapper, inner control, or detached surface.

Semantic delta

Before: accepted styling props may target the component's chosen contract element.

After (proposed): they target the same outermost DOM element on every render path.

Existing components

Popover, ContextMenu, OverflowList, Stack, PowerSearch, MultiSelector, and wrapped inputs have shapes or contracts that conflict with the proposed rule. They are not all violations of the current contract: Popover explicitly follows its current surface-styling contract. The proposal requires owners to choose migrations before the rule can become authoritative.

Scope

Specification only. No runtime behavior or public types change in this PR.

Verification

• pnpm check:knowledge
• Prettier
• git diff --check
• Independent focused review

No Changeset: specification only.