Fix display cadence refresh on monitor moves

[mstan]

Summary

• keep guest cadence separate from the host-synced effective frame period
• re-probe the SDL window current display during runtime so monitor moves or display-mode changes reapply the present cadence decision
• add a mixed-refresh cadence guard and update the PAL host-refresh guard

Fixes #278.

Notes

GitHub #278 is accurate for the reported build/logs where driver vsync could be treated as the sole cadence owner. On current master, present_should_wall_pace() already runs a wall-clock deadline cap even when driver vsync owns cadence, so this stale-display bug should not produce unbounded 200% RT there. The remaining current-master bug is stale display identity/refresh causing stale swap/present mode and a host-adjusted effective period until the runtime is restarted.

Test

• python runtime/tests/test_pal_cadence_host_refresh.py
• python runtime/tests/test_mixed_refresh_display_cadence.py
• python runtime/tests/test_present_skip_cadence_guard.py
• ctest registered guards passed for present_skip_cadence_guard_test, pal_cadence_host_refresh_test, mixed_refresh_display_cadence_test

Build verification

• Configured a minimal SDL2 runtime build with PSX_REWIND=OFF, PSXRECOMP_ALLOW_NO_BIOS=ON, and PSX_RECOMP_UI=OFF.
• Full compile was blocked by the local MSYS include environment, which failed to find SDL.h/stdlib.h before this change could be isolated further.