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>

docs/contracts/platform.md

@@ -0,0 +1,53 @@
+# Platform Contract
+
+Status: `Draft`
+
+## Purpose
+
+Define the boundary between retroDE platform integration and PS2-specific
+subsystems.
+
+## Owns
+
+- top-level clock/reset entry,
+- reset sequencing policy,
+- bridge into retroDE HPS/peripheral shell,
+- HDMI/audio adaptation boundary,
+- top-level debug/trace export path,
+- manifest/backend-visible identity plumbing.
+
+## Inputs
+
+- board clocks and resets,
+- HPS bridge traffic,
+- retroDE platform services,
+- user input events from the shared shell.
+
+## Outputs
+
+- clean subsystem clocks/resets,
+- adapted video stream,
+- adapted audio stream,
+- debug visibility path,
+- PS2-facing controller/media service inputs.
+
+## Key questions
+
+- Which subsystem clocks are generated locally?
+- Which debug signals are exported at the top level by default?
+- How much platform assistance is acceptable before the design stops being a
+  PS2 core and becomes a hybrid?
+
+## Allowed early stubs
+
+- fixed clock plan placeholders,
+- static backend identity values,
+- synthetic input injection for tests,
+- simple framebuffer-style output adapter.
+
+## Not owned here
+
+- EE memory map semantics,
+- GS packet semantics,
+- SIF semantics,
+- PS2-specific peripheral register behavior.