Replace double-casts with honest derived types in the render engine

[nathanacurtis]

Problem

Sixteen call sites coerce values through blanket double-casts, hiding real shape differences between spec types and erasing type safety exactly where the render engine meets the canvas API.

Solution

Each cast is replaced with an honest derived type or a narrow typed helper — small declared interfaces for canvas API gaps, derived types for spec-shape differences — so the compiler verifies what each call site actually relies on.

Acceptance criteria

• No double-casts remain in the package
• Spec-shape differences are modeled as named derived types

Impacted code

16 sites across packages/figma-from-specs/src/: Component, Styles primitives/composites, Elements, Effects, Subcomponents.

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Implementation details are tracked internally.

[nathanacurtis]

Implemented on feat/figma-from-specs: all 16 double-casts removed — spec-shape differences are modeled as derived types (RenderableVariant, structural Subcomponent assignability, SlotProp discriminant narrowing), and canvas-API gaps use member guards with one documented seam for by-name scalar field writes.