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/README.md

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+# Contract Docs
+
+These files define subsystem boundaries for `retroDE_ps2`.
+
+Each contract should answer:
+
+- what the block owns,
+- what enters and exits the block,
+- what timing or ordering guarantees matter,
+- what is allowed to be stubbed early,
+- what must be true before software is expected to progress.
+
+These are design contracts, not user documentation.
+
+Contract maturity levels:
+
+- `Draft`: planning-first, expected to change.
+- `Locked for Phase N`: stable enough to implement against for that phase.
+
+Current status:
+
+- All files in this folder are `Draft`.
+- Current contract set includes a dedicated interrupt-controller contract in
+  `intc.md` to keep ownership explicit across EE-visible subsystems.
+- `sio2_pad.md` is a Ch233 recon contract — no RTL yet — sketching how
+  the Ch222 HPS-side input latches will become a PS2-side `sio2_input_stub`
+  with an IOP-readable pad-state register set in a future implementation
+  chapter.