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/spu2.md

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+# SPU2 Contract
+
+Status: `Draft`
+
+## Purpose
+
+Define the audio subsystem boundary.
+
+## Owns
+
+- SPU2 register-visible state,
+- SPU2 RAM interface,
+- DMA/AutoDMA coordination,
+- audio sample generation/mixing,
+- handoff into retroDE audio output.
+
+## Inputs
+
+- IOP-side register writes,
+- DMA traffic,
+- reset/clocking controls.
+
+## Outputs
+
+- audio samples or intermediate audio stream,
+- interrupt/status signals,
+- trace events.
+
+## Questions to lock
+
+- Is any audio required before first "system boot" milestone?
+- What is the first useful milestone:
+  - register visibility only
+  - DMA playback path
+  - simple tone / RAM playback
+- Where should final resampling/adaptation to platform audio occur?
+
+## Allowed early stubs
+
+- register-visible no-audio model,
+- test-tone generator,
+- RAM playback without full SPU2 effects path.