Restrict portal shade interpolation to actual portal-edge vertices

[Copilot]

Vertices that share world coordinates across adjacent rooms could inherit lighting from the wrong room during shade matching. This showed up as bright/dark seams on non-portal geometry whenever coincident vertices happened to fall inside the portal match region.

• ### Root cause

  • Portal shade matching used a portal bounding volume check, not a portal-edge check.
  • Any coincident vertex inside that region could be marked as portal-adjacent and have its color blended with the adjoining room.

• ### Change

  • Introduced a shared PortalShadeMatchHelper used by both room compilers.
  • Replaced the broad bounding-region test with an edge-membership test: only vertices lying on the actual portal perimeter are eligible for shade matching.
  • Applied the same rule to:
    • legacy room compilation
    • TombEngine room compilation

• ### Test coverage

  • Added focused regression tests for the helper:
    • rejects vertices inside the portal surface but not on the seam
    • accepts vertices on the portal edge
  • Exposed the helper to the test assembly via InternalsVisibleTo.

// Only match shades for vertices that actually lie on the portal edge.
if (!PortalShadeMatchHelper.IsCandidate(p.Vertices, v1.Position))
    continue;