feat: dual-mode menu core with switchable native FB reader

[asturur]

Summary

Adds a runtime-switchable native FB mode to the menu core without removing any of the original menu functionality. When status[9]=0 (default), the core behaves exactly like the upstream menu — same cosine+LFSR pattern, same PAL/NTSC scandoubler timing, same HDMI wallpaper compositor, F1 wallpaper cycle still works, OSD overlays unchanged. When status[9]=1 (set by Main_MiSTer/support/zaparoo/alt_launcher.cpp), native_video_top takes over and feeds VGA from the 320×240 RGBX8888 framebuffer the launcher writes into DDR.

This is the additive counterpart to #2 — same DDR reader, same timing, but layered on top of the original menu instead of replacing parts of it.

What's carried forward from PR #2 (codex/zaparoo-rgbx8888-native-core)

• rtl/native_video_reader.sv — DDR burst reader with ping-pong buffers (control word at 0x3A000000, buf0 at 0x3A000100, buf1 at 0x3A04B100)
• rtl/native_video_timing.sv — 320×240 @ 15.734 kHz NTSC native CRT timing
• rtl/native_video_top.sv — wrapper
• PLL output1: 20 MHz → 27.027 MHz (required for the 15.734 kHz NTSC line rate)

What's deliberately not carried forward

To preserve original menu behavior:

• CONF_STR title stays "MENU" (so is_menu() in Main_MiSTer keeps matching; F1 wallpaper cycle keeps working)
• VIDEO_ARX/ARY stay 0/0 (no aspect-ratio change in cosine mode)
• Original ce_pix PAL/NTSC scandoubler logic intact
• Original cosine HV counters intact

The mux

wire mode_zaparoo = status[9];
wire use_native   = mode_zaparoo & native_active;

assign VGA_DE  = use_native ? native_de : ~(HBlank | VBlank);
assign VGA_HS  = use_native ? native_hs : HSync;
assign VGA_VS  = use_native ? native_vs : VSync;
assign VGA_R   = use_native ? native_r  : comp_v;
assign VGA_G   = use_native ? native_g  : comp_v;
assign VGA_B   = use_native ? native_b  : comp_v;

native_active = enable & frame_ready, so until the launcher fills the first DDR buffer, the cosine pattern keeps driving the output — no undriven screen on toggle.

DDR clear loop

Removed. It was one-time boot scaffolding driving DDRAM_* with zeros; native_video_reader now owns those signals. When status[9]=0 the reader holds ddr_rd=0/ddr_we=0, so DDR is unused in default mode just as before.

Test plan

• Quartus compile passes (CI build)
• Default mode on real DE10-Nano: HDMI wallpaper renders, F1 cycles wallpapers, F12 OSD works, PAL/NTSC selectable as before
• FB mode (driven by Main_MiSTer alt_launcher setting status[9]=1): 320×240 RGBX content from DDR appears on CRT and HDMI (HDMI via ascal scaling)
• Toggle back: cosine path returns, wallpaper returns

Notes for follow-up (out of scope here)

• HPS-side hotkey toggle (e.g. F2) wired into Main_MiSTer/menu.cpp to flip status[9] + video_fb_enable(0/1) + input_switch(0/1) atomically — keep tracking in a separate PR on the Main_MiSTer fork
• PAL parametrization of native_video_timing (320×240 at 50 Hz with H_TOTAL=432, V_TOTAL=312) keyed off status[4] — defer until 320×240 NTSC is validated on hardware
• 480i and other resolutions

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