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/0002-bios-policy.md

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+# Decision 0002: BIOS Policy
+
+Status: `Locked`
+
+## Context
+
+The project needed a firmware strategy that balanced authenticity with
+bring-up practicality.
+
+Main approaches considered:
+
+- real BIOS only,
+- HLE-only BIOS behavior,
+- real BIOS with narrowly-scoped debug stubs.
+
+## Options considered
+
+1. Real BIOS only.
+2. HLE-only BIOS strategy.
+3. Real BIOS plus narrow debug stubs.
+
+## Decision
+
+Adopt `real BIOS plus narrow debug stubs`.
+
+Policy details:
+
+- Real user-supplied BIOS images remain the primary firmware path.
+- Debug stubs are allowed only where they materially shorten early bring-up.
+- Stubs must be narrow, explicit, and temporary.
+
+Every stub must be tracked in a decision record or equivalent design note with:
+
+- owner,
+- purpose,
+- scope boundary,
+- removal condition.
+
+## Consequences
+
+- The project stays anchored to real PS2 boot behavior.
+- Early bring-up may proceed without waiting for every subsystem to be complete.
+- There is a maintenance cost: stub behavior must not silently become the
+  architecture.
+- The repository must never include Sony BIOS images.
+- The stub-module plan must call out which stubs are in play for each phase.