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>

sim/data/ch56_ch54_equivalent.mem

@@ -0,0 +1,102 @@
+// Ch56 external-mode real-BIOS A/B image (96 words).
+// Byte-identical to boot_install_agent_stub's built-in Ch54 image:
+//   words 0..3  = AdES handler (MFC0/ADDIU/JR/RFE)
+//   words 4..95 = 46 x (JR $31, NOP) safe-return pairs
+// Used to prove that external-mode with an identical image reproduces
+// the built-in-mode real-BIOS run (retired=986,510 / trap_pc=0x400000).
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