T9: Split-screen behind the brother-facts seam and camera FX #22

Parent

Restructure spec: #13 (RESTRUCTURE_PLAN.md)

What to build

The split-screen collapsed into one deep module whose seam is per-brother facts (position provider, visual offset, alive) instead of global brother fields and player internals; the dormant wrapper is deleted so setup happens once; camera geometry extracts into a pure core with tests; shake folds into a camera-FX adapter (the old shake autoload's inverted-scan and null-crash bugs die with it). The intentional 10px camera-offset quirk is preserved.

Acceptance criteria

• Split-screen visuals, dynamic split line, and dead-brother fallback behave exactly as they do today
• The module reads no global brother fields and no player internals
• Geometry core verified headlessly; the 10px quirk is preserved

Blocked by

• T5 Brothers, combat, and respawn through state and service (T5: Brothers, combat, and respawn through state and service #18)