T8: FlowController scene flow #21

Parent

Restructure spec: #13 (RESTRUCTURE_PLAN.md)

What to build

A single flow authority owning the phase machine and every scene transition (menu to level, level to game-over, game-over back to menu, fade intact); standalone-scene debug mode so any scene plays directly with a working session; scene loading behind a seam so the phase machine is testable headlessly; the game-over screen reads session state instead of hand-off globals.

Acceptance criteria

• Menu start, wave-complete, all-brothers-down, and back-to-menu transitions behave exactly as they do today
• Scene loading sits behind a seam; phase transitions verified headlessly in tests
• Playing any level scene directly yields a working session and flow

Blocked by

• T4 Wave and scoring through state and service (T4: Wave and scoring through state and service #17)
• T5 Brothers, combat, and respawn through state and service (T5: Brothers, combat, and respawn through state and service #18)