PIE viewport: cross-platform validation (native Linux + Windows D3D11)

[apotema]

Context

Parent: #59.

Scope

PoC validated:

• macOS native: 211 MB/s, p50=5µs raw IPC, 0 drops.
• aarch64 Ubuntu in Docker: 205 MB/s, p50=7.2ms editor-perceived, ~3% drops (container scheduler noise).

This ticket: validate on native Linux + add Windows support.

Linux native

• Run the integrated editor (post-PIE viewport: editor SHM consumer …) on an x86_64 Linux desktop with real GLFW + Mesa.
• Measure throughput / latency / drops; expect numbers comparable to macOS once container jitter is gone.
• If shm.zig needs a Linux-only branch (it didn't in Docker — fingers crossed), document why.

Windows

The PoC has no Windows code today. Two paths:

1. SHM equivalent via CreateFileMappingW — ~200 LOC of shim. Equivalent perf to the POSIX path. Ships first.
2. D3D11 shared NT handle + WGL_NV_DX_interop2 — the actual Phase 2 fast path on Windows. Producer creates a D3D11 texture with D3D11_RESOURCE_MISC_SHARED_NTHANDLE | _KEYEDMUTEX, sends handle via DuplicateHandle over named pipe. Consumer registers via wglDXOpenDeviceNV + wglDXRegisterObjectNV and gets a GL texture for free. Spout2 (open source) is essentially this code; references in the PoC research notes.

Ship 1 first; 2 is its own ticket if/when needed.

Test plan

• Native Linux run of imgui-preview-poc (or post-integration editor) reaches its own bench-equivalent numbers.
• Windows SHM path builds + runs the PoC on a Windows 11 machine.
• CI: add a Windows job to imgui-preview-poc (or the eventual integrated build) that at least builds the editor + game.

Non-goals

• Wayland — X11 first, Wayland separate.
• WSL2 — same as native Linux from the editor's perspective, no extra work.

References

• Linux Docker results: imgui-preview-poc/experiments/bench/LINUX_RESULTS.md
• D3D11 interop research: imgui-preview-poc/README.md → research summary section