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/0004-first-visible-milestone.md

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+# Decision 0004: First Visible Milestone
+
+Status: `Locked`
+
+## Context
+
+The project needed a first milestone that would produce fast, meaningful signal
+without conflating platform-video integration with EE/BIOS bring-up.
+
+## Options considered
+
+1. BIOS fetch only.
+2. GS-stub test pattern only.
+3. Minimal homebrew EE graphics.
+4. Split first milestone into two parallel proofs.
+
+## Decision
+
+Adopt a `split first milestone` with two parallel proofs.
+
+### Milestone A: platform/video proof
+
+- GS-stub test pattern through the platform video path.
+
+Purpose:
+
+- validate display plumbing,
+- validate retroDE integration,
+- isolate platform-output bugs from CPU/boot bugs.
+
+### Milestone B: core/boot proof
+
+- EE BIOS fetch and early trace match against DobieStation.
+
+Purpose:
+
+- validate memory visibility, reset vectors, and EE-side early execution,
+- isolate core bring-up bugs from display-path bugs.
+
+## Consequences
+
+- Early work can proceed in parallel on platform-video and core-boot tracks.
+- "First success" is no longer overloaded into one giant milestone.
+- Minimal homebrew EE graphics is deferred to a later integration milestone.
+- The stub-module plan should explicitly map which stubs and traces are needed
+  for Milestone A versus Milestone B.