[ALH477]

What & why

client build (Windows) has never got past configure on main. This PR fixes that and the four Windows gaps that were hiding behind it, each found by the previous fix's CI run. All of them date from 33f1ee3 (3 July), the commit that added the job. None comes from #4: it only added two steps to the Linux job, and the July runs on main died at startup on every job.

1. The toolchain argument was never expanded (9cb3734). PowerShell passes a bare -DCMAKE_TOOLCHAIN_FILE=$env:VCPKG_INSTALLATION_ROOT/... to a native command without expanding the variable. CMake received the literal text and stopped at Could not find toolchain file. The fix is to quote the argument; I reproduced both behaviours with PowerShell 7.5.4.
2. C11 atomics (c8d6034). Five files stopped at vcruntime_c11_stdatomic.h: "C atomic support is not enabled". <stdatomic.h> needs /experimental:c11atomics on top of /std:c11, which CMAKE_C_STANDARD 11 already sets. Added under if(MSVC).
3. ssize_t (c8d6034). It's POSIX, used in dm_dcf.c's sendto/recv length checks. It's now defined from SSIZE_T in src/platform/compat.h, for MSVC only (MinGW has its own).
4. GCC's x ?: y (6382cde). There were eight lua_tostring(L,-1) ? : "" in dm.viz_add_item and dm.control_add_item, a syntax error under MSVC. They are replaced by tostring_or_empty(), which evaluates lua_tostring once, as before.
5. No entry point (da11ba0). With every file compiling, the link failed on an unresolved main. On Windows SDL.h renames main() to SDL_main(), and SDL2::SDL2main supplies the entry point that calls it. It is now linked under if(WIN32 AND TARGET SDL2::SDL2main), which is vcpkg's own usage note.

Checked on Linux with this job's options (DEMOD_CLIENT, DEMOD_DCF), against Lua 5.5 (what vcpkg's classic mode now installs, not 5.4):

• -Wpedantic flags every GNU extension. Item 4's eight were the only ones in the 19 files the client compiles, and there are none now.
• The client builds with -Werror=vla (MSVC has no variable-length arrays).
• The five examples the Linux smoke runs all render.
• systems_viz and card_launcher, which call the item-4 functions, render screenshots byte-identical to those taken before the change.
• apps/mixer runs in sim mode, and luac 5.5 -p parses all 178 tracked .lua files.
• The Linux link line is unchanged by item 5.

Items 2, 3 and 5 are guarded to MSVC/Windows; item 4 compiles to the same behaviour everywhere. MSVC and its linker are measured only by this PR's Windows job, which builds and does not run the client.

Known, not changed here: on Windows MSG_DONTWAIT maps to 0 (compat.h already says so), so dm.dcf.poll() can block for up to the 200 ms receive timeout when nothing is queued. compat.h names FIONBIO as the follow-up.

Not in this PR: the Pages (browser demo) deploy fails with 404 … Ensure GitHub Pages has been enabled. The WASM build step passes. It needs Settings → Pages → Source: GitHub Actions, a repository setting.

Checklist

• Commits are signed off (git commit -s). Left to you, as on DeMoD Mixer (touch kiosk), gated DCF bridges, fixed control ops #4: a sign-off certifies the DCO in your name.
• New source files carry an SPDX-License-Identifier header. N/A: no new files.
• make and make test pass. Items 2, 3 and 5 are Windows-only; item 4 was checked by screenshot identity, and the Linux job re-runs the suite.
• No vendored code was relicensed, no generated artifacts committed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YEapehGd6iN1d5vu53PEN7