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Question
Could the amplituhedron describe what survives a scattering interface without exhausting the vacuum?
What the note explains
Linearized admissibility, exact finite continuation, and complete reconstruction are separate requirements. The generic complex two-leg tangent plane is two-dimensional; exact affine null preservation selects two chiral lines.
Example
Impose momentum conservation and linearized mass-shell preservation first. Then add the quadratic null condition. The second step removes mixed directions even though they passed the linear test.
What the approach gains
The interface viewpoint distinguishes exact finite geometry from completeness of the source description. The split-cone toy model provides a precise algebraic comparison and exposes the additional physical realization still needed.
Reading route
Read the two-channel calculation before the source analogy. Then follow the bridge conditions and all thirteen open questions; the physical source construction is the research task, not an established consequence.
Scope and limits
The earlier source ZIP has a shorter 10-page body and cannot reproduce this 12-page revision. The later PDF is preserved in full; only two clipped display regions are repaired in the production copy.