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/decisions/0003-golden-reference.md

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+# Decision 0003: Golden Reference Strategy
+
+Status: `Locked`
+
+## Context
+
+The project needs software references for:
+
+- early boot and subsystem trace comparison,
+- broader behavior cross-checking,
+- dispute resolution when implementations differ.
+
+The main candidates were DobieStation, PCSX2, Play!, or a custom-purpose
+reference.
+
+## Options considered
+
+1. DobieStation as sole primary reference.
+2. PCSX2 as sole primary reference.
+3. Multiple references with role separation.
+4. Play! as primary reference.
+5. Purpose-built minimal reference.
+
+## Decision
+
+Adopt `multiple references with role separation`.
+
+Role split:
+
+- DobieStation: early boot and subsystem-oriented trace comparison.
+- PCSX2: breadth and behavior cross-checking.
+- Sony documentation and authoritative hardware references: tiebreak when
+  available.
+
+This project does not treat PCSX2 as a mere fallback; it is a peer reference
+used for a different purpose.
+
+## Consequences
+
+- Validation infrastructure must be able to normalize traces from more than one
+  reference source.
+- Some disagreements will require manual triage rather than simple "reference
+  wins" logic.
+- The project gets a smaller, more coherent early trace oracle while retaining
+  access to broader real-world emulator behavior.
+- The validation contract and stub-module plan should name which reference is
+  used for each milestone.