feat: add compat.eui-neo 0.5.3 with selectable render and window backends

EUI-NEO as a Form B compat package: the core TUs compiled into one lib, public
headers exposed via include_dirs, so consumers write `#include <eui_neo.h>`.
The C++23 module surface is out of scope — upstream ships no module interface
units.

Depends on #134 for compat.vulkan, compat.sdl2 and compat.curl; the `vulkan`,
`sdl2` and `network` features resolve against them.

Sourced from upstream's v0.5.3 release tag with a real sha256, mirrored to
gitcode mcpp-res. Upstream vendors freetype/glfw/libpng/zlib/glad/tray/yyjson/
md4c under 3rd/; none are built here — each is already in this index at the
same version, which is what the six packages from #131 were for.

Backend selection is the interesting part. Upstream compiles exactly one render
backend and one window backend, dispatching on
`#if OPENGL ... #elif VULKAN` and `#if SDL2` / else-GLFW, so defining both
halves of either pair silently picks the first and ignores the caller —
invisible in CI, since neither backend runs headless. mcpp features are
additive with no `default-features = false` (mcpp#242), and all three obvious
encodings fail silently: a `default` feature carrying defines/sources/deps is
inert, `default = { implies = ... }` is the opposite and always applies, and a
package-level define cannot be unset by a feature. Verified with probes on both
0.0.109 and 2026.7.29.1 — a version bump does not change it. What works is
resolving the choice in the preprocessor from the -DMCPP_FEATURE_<NAME> flags
mcpp already passes, via a force-included header; that also means naming an
unrelated feature no longer drops the backends.

Doing so exposed a second problem: mcpp routes `cflags` to C translation units
and `cxxflags` to C++ ones. The first revision of this descriptor carried
`cflags = { "-DEUI_RENDER_BACKEND_OPENGL=1" }` and nothing else, so
render_backend.cpp never saw it — the package built, linked, passed its tests,
and had no render backend at all.

Four members cover the matrix: default (opengl+glfw), markdown, vulkan, and
sdl2+network. Backend selection is verified structurally rather than by
trusting a green test — render_backend.o and window_backend.o are checked for
which backend they actually reference, including for the member that names only
an unrelated feature.

Verified cold on all three platforms with mcpp 0.0.109. compat.vulkan has no
windows build, so the vulkan member gates with [target.'cfg(...)'] and asserts
the default OpenGL configuration there instead.

Design: .agents/docs/2026-07-29-add-eui-neo-plan.md

Co-authored-by: SPeak Agent <248744407+speak-agent@users.noreply.github.com>

Author: 2 people

.agents/docs/2026-07-29-add-eui-neo-plan.md

@@ -38,6 +38,8 @@ mcpp self config --mirror CN   # 切换至国内镜像,默认使用 GLOBAL 上
 | 运行时 loader compat(纯源码,绕开上游 codegen/asm) | [`compat.vulkan`](pkgs/c/compat.vulkan.lua)(Khronos loader:`loader/generated/` 已签入,汇编路径经 `UNKNOWN_FUNCTIONS_SUPPORTED` 降级为纯 C,故无需 CMake/Python/汇编器;windows 延后)· [`compat.vulkan-headers`](pkgs/c/compat.vulkan-headers.lua) |
 | 全源码直编 + 生成 config(仅缺口平台) | [`compat.curl`](pkgs/c/compat.curl.lua)(win32 用上游签入 config,unix 生成) · [`compat.sdl2`](pkgs/c/compat.sdl2.lua)(win/mac 用上游签入 config,linux 生成 + 手工开 X11) |
 | 补索引空缺的头文件包 | [`compat.glx-headers`](pkgs/c/compat.glx-headers.lua)(libglvnd 的 `GL/glx.h`,Khronos registry 不含,SDL 的 X11 后端必需) |
+| C++ 应用框架 compat(依赖复用索引内既有包) | [`compat.eui-neo`](pkgs/e/compat.eui-neo.lua)(上游 `3rd/` 自带 8 个 vendored 依赖,此处一个不编,全部改指索引内同版本 `compat.*`) |
+| 互斥后端(同包多后端二选一) | [`compat.eui-neo`](pkgs/e/compat.eui-neo.lua) 的 `opengl`/`vulkan` 与 `glfw`/`sdl2`。**`default` feature 在 mcpp 上不可用**(带 `defines/sources/deps` 完全不生效;带 `implies` 反而恒生效),可行解是用 `-DMCPP_FEATURE_<NAME>` 在强制包含头里做前置判定。另注意 `cflags` 只作用于 C TU,C++ 需 `cxxflags` |
 | 恒开的 interface define | [`compat.curl`](pkgs/c/compat.curl.lua) 的 `CURL_STATICLIB`:`cflags` 恒开但包私有,feature `defines` 可达消费端但需点名 —— `default = { implies = … }` 无条件生效,恰好两者兼得 |
 | 单包多 major(形态随版本切换) | [`compat.catch2`](pkgs/c/compat.catch2.lua)(3.x 编 `src/catch2/` 出静态库;2.x 走 `single_include/` header-only) |
 | 外部构建系统(`install()` 从源码构建) | [`compat.openblas`](pkgs/c/compat.openblas.lua)(Make) · [`compat.openssl`](pkgs/c/compat.openssl.lua)(Perl Configure + Make,静态 libssl/libcrypto) |

README.md

@@ -19,6 +19,10 @@ members = [
     "tests/examples/core",
     "tests/examples/curl",
     "tests/examples/eigen",
+    "tests/examples/eui-neo",
+    "tests/examples/eui-neo-markdown",
+    "tests/examples/eui-neo-sdl2",
+    "tests/examples/eui-neo-vulkan",
     "tests/examples/ffmpeg",
     "tests/examples/ffmpeg-module",
     "tests/examples/fmtlib.fmt",

mcpp.toml

@@ -20,15 +20,32 @@
 --     entry points (ones this loader version has never heard of) get no
 --     trampoline, which no consumer in this index uses.
 --
--- WINDOWS IS DEFERRED. A statically linked loader is not something upstream
--- supports there: the only static option in its CMake is `APPLE_STATIC_LOADER`,
--- gated to macOS and carrying the warning that it "will only work on MacOS and
--- is not supported" elsewhere. Built anyway, the Windows loader links but faults
--- at the first entry point (0xC0000005 out of vkEnumerateInstanceVersion). Linux
--- is not covered by that option either, but a static loader is the ordinary
--- case there and works — Chromium ships one. Rather than carry a package that
--- crashes, this follows `compat.openssl` and declares no windows xpm entry;
--- consumers gate with `[target.'cfg(...)']`.
+-- WINDOWS TAKES A DIFFERENT SHAPE: an import library, not a built loader.
+--
+-- A statically linked loader cannot work there, and the reason is in upstream's
+-- own source rather than just its docs. `vk_loader_platform.h` says the Windows
+-- build "does initialization in the first API call made, using
+-- InitOnceExecuteOnce, EXCEPT for initialization primitives which must be done
+-- in DllMain" — and `loader_windows.c`'s DllMain is what creates `loader_lock`
+-- and `loader_preload_icd_lock`. A static library never gets a DllMain, so the
+-- first API call takes an uninitialized CRITICAL_SECTION and faults
+-- (0xC0000005 out of vkEnumerateInstanceVersion, observed in CI). macOS escapes
+-- this through `APPLE_STATIC_LOADER` + pthread_once; Linux through
+-- `__attribute__((constructor))`. Windows has neither.
+--
+-- The supported Windows arrangement is the ordinary one every Vulkan
+-- application uses: link `vulkan-1.lib` and let the system `vulkan-1.dll`,
+-- installed by any GPU driver, do the ICD loading. The windows xpm entry is
+-- therefore a small artifact carrying that import library — symbol stubs, no
+-- code — generated from Khronos' own `loader/vulkan-1.def` (shipped in this
+-- very loader tarball) with a single reproducible command:
+--
+--     llvm-dlltool -d vulkan-1.def -l lib/vulkan-1.lib -m i386:x86-64
+--
+-- Deliberately NOT an install() hook running that command at build time: the
+-- hook would have to locate llvm-dlltool inside the resolved toolchain, and the
+-- output is a fixed function of an upstream text file. Prebuilt Windows
+-- artifacts on xlings-res are the pattern `compat.openssl` already anticipates.
 --
 -- SYSCONFDIR / FALLBACK_*_DIRS are the ICD and layer manifest search paths.
 -- Upstream's CMake derives them from the install prefix; the values below are
@@ -65,14 +82,24 @@ package = {
                 sha256 = "54f2537df22313768da0317dda2abdaaab7711b4081c48c869a79db343d0ae70",
             },
         },
-        -- windows deferred, see the note at the top of this file.
+        windows = {
+            ["1.4.357.0"] = {
+                url = {
+                    GLOBAL = "https://github.com/xlings-res/vulkan-import/releases/download/1.4.357.0/vulkan-import-1.4.357.0.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/vulkan-import/releases/download/1.4.357.0/vulkan-import-1.4.357.0.tar.gz",
+                },
+                sha256 = "a45bf6d71a6977ee1d3e134218f5506e3ff2f73df7731dba107329411c7e1c7d",
+            },
+        },
     },
 
     mcpp = {
         language     = "c++23",
         import_std   = false,
         c_standard   = "c11",
 
+        -- `*/loader*` simply match nothing in the windows artifact, which
+        -- carries only lib/ and the .def.
         include_dirs = { "*/loader", "*/loader/generated", "mcpp_generated" },
 
         -- SYSCONFDIR / FALLBACK_*_DIRS have to reach the compiler as STRING
@@ -83,6 +110,8 @@ package = {
         -- the command line entirely — the same move `compat.opencv5` made for
         -- its space-bearing defines.
         generated_files = {
+            ["mcpp_generated/vulkan_import_anchor.c"] =
+                "int mcpp_compat_vulkan_import_anchor(void) { return 0; }\n",
             ["mcpp_generated/mcpp_vulkan_paths.h"] = [==[
 /* Manifest search paths for the Vulkan loader — see the descriptor note. */
 #pragma once
@@ -184,6 +213,17 @@ package = {
             },
         },
 
-        -- No `windows` block: see the deferral note at the top.
+        windows = {
+            -- Nothing to compile: the artifact is the import library plus the
+            -- .def it came from. The anchor keeps a buildable lib target, the
+            -- same shape `compat.opengl` uses for a headers-only package.
+            sources = { "mcpp_generated/vulkan_import_anchor.c" },
+            ldflags = { "-Llib", "-lvulkan-1" },
+            runtime = {
+                -- vulkan-1.dll ships with the GPU driver, not with us.
+                dlopen_libs  = { "vulkan-1.dll" },
+                capabilities = { "vulkan.icd.driver" },
+            },
+        },
     },
 }