Ownership-safe native analytic shadow pipeline

[micahcooley]

Scope

• Native fill_rounded_shadow display-list operation.
• Deterministic fixed-point CPU shadow rasterization and codec transport.
• Preserve the newer ownership-safe visual API and retained Surface contract.
• Replace SurfaceV2's 4 ambient + 3 contact approximation bands with one ambient + one contact operation.
• Follow-up in this same integration line: identity-only hit/semantics emission so SurfaceV2 no longer paints legacy chrome underneath modern chrome.

Measurable targets

• Resting solid panel: reduce retained display work from roughly 13 operations to 5.
• Resting transient glass overlay: reduce roughly 14 operations to 6.
• No hidden allocation, Gaussian temporary image, recursive blur, or parallel accessibility ID.
• CPU output deterministic; GPU backends consume the same render IR.

Current gate

The stale compiler pin was advanced from 43870455 to named-owner revision 22f8c1f4. Strict validation now reaches only the upstream aggregate-let lowering defect in flex_item_intrinsic and flex_layout; that repair is tracked in Sylorlabs/zag#3. Renderer code is not promoted until the compiler fixed point and the complete SurfaceV2/headless contracts pass.

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