Cross-site brand inputs (--design-source / --brand-source)

Consumed by stardust:direct Setup step 3b when extract captured brand signal beyond the primary origin.

Two extract-written signals widen the brand surface beyond the primary origin:

• state.json.designSource — design-donor mode (extract ran with --design-source <url>). The donor's derived system at stardust/canon-source/DESIGN.{md,json} becomes the target the Mode A pins bind to: palette and type pin to the donor surface while content, IA priorities, and per-page evidence stay with the primary origin. Surface in the plan: "Design-donor mode — target system inherited from ."
• _brand-extraction.json.origins[] — sibling-property evidence captured via --brand-source. Traits captured on a sibling origin count as captured evidence for trait amplification (variant B/C briefs, improvements items) exactly like primary-origin evidence; cite the origin in the evidence citation. Under Mode A the pins still come from the primary origin unless designSource says otherwise — sibling evidence widens what can be amplified, not what is pinned.