P1: Split oversized `OpenElement` class

[SisyphusZheng]

What to build

packages/element/src/open-element.ts is 681 lines and handles styles, themes, DSD/CSR, lifecycle, signals, params, forms, and i18n. Split into focused helpers or mixins.

Acceptance criteria

• open-element.ts is under 400 lines.
• New helpers (e.g., ThemeManager, HydrationScope) are unit-testable.
• All element tests pass.

Blocked by

None