Document the PCSX2 test setup

PCSX2 was set up and working but existed only in this machine's filesystem
and in people's heads, so every comparison looked like a one-off observation
rather than something re-runnable. The setup section in
docs/plan/pcsx2-verification.md now records where the emulator, the BIOS, the
disc, the savestates and the PINE socket are, how a state is captured, and
which command the disc integration tests need -- and AGENTS.md and README.md
point at it from their orientation sections. One operational detail worth
spelling out: the emulator's directory is lowercase (pcsx2), which is not
what people type.

README.md

@@ -88,6 +88,12 @@ Visual and behavioural comparison against PCSX2, replaying recorded input and
 comparing frames, level by level. The PS2 float model is reproduced rather than
 approximated — see [`AGENTS.md`](AGENTS.md#2-reproduce-the-ps2-float-model).
 
+PCSX2 is installed and working on this machine; the comparison loop that reads
+the game's own memory over its debug protocol is up, and level 0's and level
+1's tfrag lighting are verified byte-exact against it. See
+[`docs/plan/pcsx2-verification.md`](docs/plan/pcsx2-verification.md) for the
+setup and how to run it.
+
 ## Credits and licensing
 
 This project learns from and is built on the work of others. Licences are