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/0001-posture.md

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+# Decision 0001: Project Posture
+
+Status: `Locked`
+
+## Context
+
+The project needed to choose between:
+
+- native full-system on current hardware,
+- staged subset on current hardware,
+- hybrid architecture,
+- future-hardware target.
+
+This decision sets the planning posture for all early contracts and milestones.
+
+## Options considered
+
+1. Native full-system on current hardware.
+2. Staged subset on current hardware.
+3. Hybrid architecture with significant host-side execution.
+4. Future-hardware target.
+
+## Decision
+
+Adopt `staged subset on current hardware`.
+
+This means:
+
+- the project targets the current retroDE platform,
+- the architectural path remains that of a real PS2 core,
+- coverage will be incomplete for an extended period,
+- early phases prioritize observable bring-up over broad software
+  compatibility.
+
+The intended interpretation is:
+
+`real PS2 architectural path, incomplete coverage`.
+
+## Consequences
+
+- Early milestones may validate only parts of the machine.
+- Some subsystems can remain stubbed or reduced while others become real.
+- The project preserves continuity with the rest of the retroDE family by
+  staying on current hardware.
+- The project explicitly does not promise full-title compatibility on the
+  current platform.
+- Contracts and milestones should optimize for progressive integration instead
+  of "all-or-nothing" completeness.