Add PC port scaffolding (CMake build + port layer)

Initial scaffolding for a PC port of GoldenEye 007, modeled on the
Perfect Dark PC port (same Rare "Indy" engine). The N64 build
(Makefile + IDA) is untouched; the PC build is a parallel CMake build.

Build:
- CMakeLists.txt: SDL2 + zlib + OpenGL. Compiles src/game/*.c + curated
  engine files + libultra audio/gu + port/. Per-region macro sets
  (ROMID US/EU/JP) mirror the N64 Makefile's LCDEFS exactly.
- build-pc.sh, cmake/: configure+build helper and target/SDL2 modules.

Port layer (port/):
- video/audio/input/fs/romdata/config/system/mixer: stubs with
  TODO(Phase N) markers and correct interfaces.
- libultra.c: libultra OS shims (message queues, VI, PI, AI, EEPROM,
  controller, PFS/motor no-ops, region-gated osTvType).
- gesched.c: software-RSP scheduler replacing src/sched.c task submission.
- n64stubs.c: inert stubs for N64-boot + rmon symbols.
- random.c: PRNG ported verbatim from src/random.s (bit-exact).
- ucode.c: RSP/ASP/GSP microcode segment markers (dummies; Phase 3 TODO).

Docs:
- docs/PCPortResearch.md: architecture, GE-specific deltas, phased plan,
  and the scaffolding review (findings A1-A4/B1-B4/C1-C2 + resolutions).
- PORTING.md: top-level entry point.

See docs/PCPortResearch.md for the full plan.

build-pc.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+#
+# Build the GoldenEye 007 PC port.
+#
+# Usage:
+#   ./build-pc.sh [ntsc-final|pal-final|jpn-final]
+#
+# Dependencies (see docs/PCPortResearch.md):
+#   - CMake >= 3.16
+#   - SDL2 dev
+#   - zlib dev
+#   - OpenGL dev (opengl32 on Windows, GL on Linux, OpenGL.framework on macOS)
+#
+# Example (Linux):
+#   sudo apt install cmake libsdl2-dev zlib1g-dev libgl4-mesa-dev
+# Example (macOS):
+#   brew install cmake sdl2 zlib
+# Example (Windows/MSYS2):
+#   pacman -S mingw-w64-x86_64-toolchain mingw-w64-x86_64-SDL2 \
+#             mingw-w64-x86_64-zlib mingw-w64-x86_64-cmake
+#
+set -euo pipefail
+
+ROMID="${1:-ntsc-final}"
+BUILD_DIR="build-pc"
+
+echo "==> Configuring PC port (ROMID=${ROMID})"
+cmake -S . -B "${BUILD_DIR}" -DROMID="${ROMID}"
+
+echo "==> Building"
+cmake --build "${BUILD_DIR}" -j
+
+echo "==> Done."
+echo "    Binary: ${BUILD_DIR}/ge007.*"
+echo "    Put your ROM in ./data/ (see README) and run the binary."