[mstan]

The merged R1 background-source span implementation remained OFF in ordinary builds. Enable it by default for the qualified Windows x64 NATIVE configuration, preserving explicit cache choices and the other-platform/LLE floors. Keep the current supported engine/UI pins.

Fresh adaptive Auto windows also started at 4:3 before the widescreen plugin activated. Read the already-committed feature/option selection for initial sizing, while preserving saved native, fixed-aspect and custom-window choices and the existing plugin lifecycle.

Validation completed:

• Five existing renderer/reference suites and current-build save/load, rewind, fresh-process replay and varying-width/count rollback passed.
• Three alternating 4,200-frame Highway pairs at fixed 16:9 reduced process CPU by 11.09%, 21.33% and 20.76%; sampled gameplay images and native-line counts matched. Stock controls were neutral/inconclusive under variable background load.
• Fresh defaults created a 1281x720 window with 342x224 gameplay. Saved native and 800x600 custom controls retained their sizes; the separate raster-LLE build produced the same sampled pixels.
• Run-ahead preserved the final WRAM/CPU trace and showed the exact next reference picture. The existing host profile leaves 1.422 ms/frame of inclusive capture/native-field producer wall time; it includes preemption and is not isolated copy cost.

The performance samples use SDL3 dummy/software presentation and measure CPU work, not GPU timing or hardware FPS. The startup correction is qualified separately from the fixed-16:9 span A/B.

The repository build check passes. Normal integration/release checks remain; no framework upgrade or new span implementation is included.