Initial commit: retroDE_ps2 — first-of-its-kind PS2 GS FPGA core (DE25-Nano / Agilex 5)

RTL (GS rasterizer, EE core stub, platform bridge, LPDDR4B path), sim regression
(272 TBs), docs, and tooling. Copyrighted PS2 content (BIOS, game code, GS dumps,
and all dump-derived textures/traces) is excluded via .gitignore and stays local.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

rtl/platform/README.md

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+# rtl/platform
+
+retroDE-specific platform integration. Matches `docs/contracts/platform.md`.
+
+## Wave 1 contents
+
+- `platform_video_stub.sv` — free-running raster generator. Default VGA
+  640x480 timing (overridable per-testbench to tiny values for fast sim).
+  Takes `bg_{r,g,b}` from `gs_stub` and flood-fills the active region.
+  Emits one `EV_MODE` per completed frame so testbenches can count frames
+  without sampling raw video.
+
+## Scope boundary
+
+This directory owns:
+
+- clock/reset sequencing entry points,
+- retroDE-facing video and audio adaptation,
+- HPS bridge plumbing (future),
+- top-level wrappers not belonging inside PS2 subsystems.
+
+It does **not** own GS/PCRTC semantics (that's `rtl/gif_gs/`), SPU2 audio
+synthesis (`rtl/spu2/`), or any PS2 register behavior.
+
+## Replacement path
+
+`platform_video_stub` stays as the platform adaptation layer. What changes
+is the upstream pixel source: Wave 1 → flat BGCOLOR from `gs_stub`,
+later waves → fuller GS/PCRTC output including framebuffer scan-out.