docs+patch: psx-beetle oracle on Linux (pinned beetle-psx base + SIO trace hook)

beetle_libretro.cpp targets the C++ beetle-psx tree; upstream master has
been converted to plain C and no longer compiles against it. Documented
the working recipe: pin beetle-psx at 5759277b (the wtrace patch's base,
located via the patch's own blob hash), apply beetle_wtrace_hook.patch,
and apply the new beetle_sio_trace_hook.patch — FrontIO::SetSIOTraceCallback
(fires once per completed SIO byte exchange), a hook beetle_libretro.cpp
required that was missing from the committed patch set.

Verified on Linux/Xvfb: psx-beetle boots Kula World to demo gameplay and
serves ping/screenshot over the shared TCP debug protocol, alongside
kula-runtime — the two-process oracle setup from CLAUDE.md §16 now works
in this environment. Runtime-created per-game .mcr files are gitignored.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BdjxoRB1f9VQsNzZayCgGS

.gitignore

@@ -84,3 +84,7 @@ _wip/
 games/*/*
 !games/*/game.toml
 !games/*/seeds.txt
+
+# Per-game memcards created at runtime
+*.mcr
+!dummy.*.mcr

docs/beetle-linux.md

@@ -0,0 +1,41 @@
+# Building psx-beetle on Linux
+
+The psx-beetle oracle builds and runs on Linux with the same wire protocol
+as on Windows. Verified 2026-07-02 (Kula World boots to demo gameplay;
+ping/screenshot served over the TCP debug protocol).
+
+## beetle-psx checkout
+
+`runtime/src/beetle_libretro.cpp` targets the C++ beetle-psx tree. Upstream
+master has since been converted to plain C (`libretro.cpp` -> `libretro.c`,
+`PS_CPU` class dropped), which no longer compiles against our integration.
+Pin the checkout to the last compatible commit — the same base
+`docs/beetle_wtrace_hook.patch` was generated from:
+
+```bash
+git clone https://github.com/libretro/beetle-psx-libretro.git beetle-psx
+cd beetle-psx
+git checkout 5759277b          # "audit pass" — last C++-tree base we target
+patch -p1 < ../docs/beetle_wtrace_hook.patch
+patch -p1 < ../docs/beetle_sio_trace_hook.patch
+```
+
+`beetle_sio_trace_hook.patch` adds `FrontIO::SetSIOTraceCallback` (fires per
+completed SIO byte exchange) — an integration hook beetle_libretro.cpp needs
+that predated the committed wtrace patch.
+
+## Build
+
+```bash
+# Static lib. On unix the artifact is named mednafen_psx_libretro.so but is
+# an ar archive (STATIC_LINKING=1); stage it under the name cmake expects.
+make platform=unix STATIC_LINKING=1 HAVE_LIGHTREC=0 -j"$(nproc)"
+cp mednafen_psx_libretro.so libmednafen_psx.a
+
+cd ../runtime
+cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DPSX_LAUNCHER=OFF -DPSX_DEBUG_TOOLS=ON
+ninja -C build psx-beetle
+
+# Headless run (picks the next free port from 4380 if taken; watch stderr)
+xvfb-run -a ./runtime/build/psx-beetle ../bios/SCPH1001.BIN --disc <game.cue>
+```

docs/beetle_sio_trace_hook.patch

@@ -0,0 +1,35 @@
+diff --git a/mednafen/psx/frontio.cpp b/mednafen/psx/frontio.cpp
+index a35b022f..02e7ce0b 100644
+--- a/mednafen/psx/frontio.cpp
++++ b/mednafen/psx/frontio.cpp
+@@ -783,6 +783,10 @@ int32_t FrontIO::Update(int32_t timestamp)
+                   ReceiveInProgress = false;
+                   ReceiveBufferAvail = true;
+ 
++                  /* PSXRecomp oracle hook: byte exchange complete. */
++                  if(MDFN_UNLIKELY(sio_trace_cb))
++                     sio_trace_cb(TransmitBuffer, ReceiveBuffer, (uint16_t)Control);
++
+                   if(Control & 0x800)
+                   {
+                      istatus = true;
+diff --git a/mednafen/psx/frontio.h b/mednafen/psx/frontio.h
+index 8a67f5af..d9a260ca 100644
+--- a/mednafen/psx/frontio.h
++++ b/mednafen/psx/frontio.h
+@@ -95,8 +95,15 @@ class FrontIO
+ 
+       int StateAction(StateMem* sm, int load, int data_only);
+ 
++      /* PSXRecomp oracle hook: fires once per completed SIO byte exchange
++       * (tx byte sent, rx byte received, current Control register). */
++      typedef void (*sio_trace_cb_t)(uint8_t tx, uint8_t rx, uint16_t ctrl);
++      void SetSIOTraceCallback(sio_trace_cb_t cb) { sio_trace_cb = cb; }
++
+    private:
+ 
++      sio_trace_cb_t sio_trace_cb = nullptr;
++
+       void DoDSRIRQ(void);
+       void CheckStartStopPending(int32_t timestamp, bool skip_event_set = false);
+