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/peripherals.md

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+# Peripheral Contract
+
+Status: `Draft`
+
+## Purpose
+
+Group the console-completeness devices that are neither CPU cores nor the main
+graphics/audio engines.
+
+## In scope
+
+- CDVD
+- SIO2
+- memory cards
+- controller-facing console semantics
+- DEV9
+- USB
+- FireWire
+
+## Owns
+
+- register-visible behavior for these devices,
+- media/card/controller presence semantics,
+- protocol translation where the retroDE platform provides host assistance.
+
+## Questions to lock
+
+- Which peripherals are required for the first three milestones?
+- Which peripherals will be HPS-assisted versus locally modeled?
+- Is controller input presented first through a simplified abstraction or
+  through SIO2-faithful transactions?
+
+## Allowed early stubs
+
+- device-present/device-absent reporting only,
+- fixed media status responses,
+- controller event injection through simplified paths,
+- memory card placeholder presence with no persistence.
+
+## Likely implementation order
+
+1. SIO2/controller minimum
+2. memory card minimum
+3. CDVD minimum
+4. DEV9/USB/FireWire as later completeness work