ci: add per-PR cross-platform build gate

[Zorkats]

What

Adds .github/workflows/build.yml — a build-only CI workflow that compiles the ssb64 target on Windows (MSVC), Linux (clang), and macOS (arm64) for every pull_request and push to main.

Why

Today the only workflow is release.yml, which runs solely on v* tags. No check verifies that a contribution even compiles across platforms before review or merge — a maintainer has to build it by hand on three machines to find out. That's a real gap with active cross-platform work in flight (e.g. a mod loader using a Windows-only detour primitive would pass review and silently break the macOS arm64 release target).

This gate catches that automatically, in red, on the PR.

Design

• Build-only. Never packages an installer, never produces BattleShip.o2r. Like release.yml, it's ROM-independent end-to-end, so it's safe for public CI.
• Mirrors release.yml's desktop jobs minus packaging, so a green run here predicts a green release build:
  • Same find_package deps (libzip, nlohmann_json, tinyxml2, spdlog, SDL2, GLEW) + Pillow for the unconditional CSS scroll-arrow codegen.
  • fetch-depth: 0 so the updater's git describe --tags resolves at configure time.
  • On Windows, Python3_EXECUTABLE is pinned to the same python that pip installed Pillow under, avoiding the multi-interpreter "Pillow is required" failure.
• No manual codegen presteps. Credits text, CSS arrows, reloc tables, and the embedded icon are all driven by CMake custom targets/commands as build dependencies — the workflow only ensures their tools (Python3 + Pillow) are present.
• concurrency cancels superseded runs on the same ref to save runner minutes.

Notes for review

• Builds the us version only (default). The us/jp matrix in release.yml is release-scoped; a single region is enough to catch compile breakage.
• macOS runner minutes carry a higher multiplier — flagging it in case you'd prefer the macOS job gated to non-draft PRs or main only. Easy to adjust.