Source-Owned Buffering Indicator

[mihar-22]

Parent epic: #1949

Context

Packaged Default and Minimal Video Skins compose a spinner-backed buffering indicator. Canonical source currently has no complete buffering composition, so generated output cannot match loading-state markup and styles.

The behavior primitive already exists in Core, React, and HTML. This issue owns the complete reusable Video Skin component and its component-owned styles. ErrorDialog is tracked separately in #2188.

Acceptance criteria

• Canonical BufferingIndicator composes the existing behavior primitive and spinner without platform intrinsics.
• Default and Minimal reuse the same component where behavior is identical.
• React and HTML output is idiomatic and uses public imports or registrations.
• Tailwind and vanilla CSS preserve rendered presence, animation, reduced-motion behavior, and accepted visual differences.
• The component is independently installable with exact component, icon, and style closure.

Final interaction, accessibility, and visual validation belongs to #2183.