Add eui neo 0.5.5

[FarnaHerry]

Summary

Bump compat.eui-neo from 0.5.3 to 0.5.5. Upstream 0.5.4/0.5.5 are hotfix
releases, but 0.5.5 grew a Shadertoy subsystem that is mandatory in the
core library — so this is more than a version-entry bump.

Changes

Descriptor (pkgs/e/compat.eui-neo.lua):

• Add 0.5.5 xpm entries on linux/macosx/windows (0.5.3 retained).
  sha256 cf0da91d…eb98, computed twice from the GLOBAL tarball.
• Base sources 20 → 24: core/render/shadertoy.cpp, shadertoy_json.cpp,
  shadertoy_primitive.cpp, core/render/opengl/opengl_shadertoy.cpp.
  Required, not optional: render_backend.h and include/eui/types.h
  include core/render/shadertoy.h unconditionally, and opengl_backend.cpp
  calls releaseShaderToys() — the old 20-TU recipe fails to link.
• vulkan feature gains vulkan_shadertoy.cpp.
• -fno-char8_t moved from the windows profile to package-wide cxxflags:
  0.5.5's resolveResourcePath() (platform.cpp:616) and the Shadertoy TUs
  return path::u8string() as std::string on every platform. The 0.5.3
  char8_t break was Windows-only; CI caught the all-platform regression on
  linux + macos.

Test members: all six tests/examples/eui-neo* bumped to 0.5.5 so the
whole feature surface (markdown / vulkan / sdl2+network / app-main / window)
exercises the new version.

Docs: .agents/docs/2026-08-05-add-eui-neo-0.5.5-plan.md.

CN mirror

0.5.5 uses the plain-string (GLOBAL) url fallback per docs/cn-mirror.md —
no mcpp-res write access to publish the gitcode release from this fork yet.
Flip to { GLOBAL, CN } once the mirror exists (sha256 unchanged).

Verification

• mcpp xpkg parse OK: 0.5.3+0.5.5 on all platforms, sources 24, features 6.
• sha256 computed twice from the GLOBAL tarball (byte-identical, 13,339,545 B).
• Vendored 3rd/ deps confirmed unchanged vs the descriptor pins (freetype
  2.13.3, glfw 3.4, libpng 1.6.43, zlib-1.3.1, yyjson-0.12.0).
• CI (linux / macos / windows, mcpp 2026.8.3.3): all green — the six
  eui-neo members build and pass on all three platforms (selective member
  testing, not a full-workspace run).

[FarnaHerry]

@Sunrisepeak @Aimol-l