Adds test_detect_vram_preflight_fallback to cover the spec path where
cf-orch is unreachable but scripts.preflight.get_gpus() succeeds,
verifying detect_available_vram_gb() returns the summed total VRAM.
Uses sys.modules injection to simulate the preflight module being present.
Extract generic batch scheduler into circuitforge_core.tasks.scheduler
so any CircuitForge product can use it. Includes VRAM detection via
cf-orch coordinator (cooperative free-VRAM), preflight fallback, and
unlimited fallback; singleton API; full test coverage (12 tests).
