feat(pkgs): add cJSON (compat) + nlohmann.json (C++23 module) + per-pkg CI

[Sunrisepeak]

What

Adds two third-party JSON libraries to the index, plus a per-changed-package CI mechanism.

compat.cjson @ 1.7.19

Pure-C source build, same shape as compat.zlib: compiles cJSON.c into a lib, exposes cJSON.h via include_dirs. The optional cJSON_Utils extension is gated behind feature utils (mirrors how compat.gtest gates gtest_main behind main).

#include <cJSON.h>   // header self-guards with extern "C"

nlohmann.json @ 3.12.0 → import nlohmann.json; out of the box

Released v3.12.0 is header-only and ships no module interface unit. Upstream has authored an official one at src/modules/json.cppm (export module nlohmann.json;) but it lives on develop only (not in any release tag yet). So this package provides it ourselves via mcpp generated_files, embedding upstream's official json.cppm verbatim (literals + the MSVC #3970 detail re-exports); base headers stay pinned to the reproducible v3.12.0 tarball.

import std;
import nlohmann.json;
using namespace nlohmann::literals;   // for the _json UDL

Idiomatic: pair with import std;. Don't mix the module import with textual #include <string> — GCC's modules implementation clashes on the std headers the module already attaches. When upstream ships the .cppm in a release, switch sources to */src/modules/json.cppm and drop generated_files (noted in the descriptor).

CN mirrors

Both tarballs mirrored to GitCode via tools/gtc, byte-identical to source (sha256 verified):

• mcpp-res/cjson 1.7.19 → CN url 200
• mcpp-res/nlohmann-json 3.12.0 → CN url 200

Per-changed-package CI

tests/examples/<pkg>/ holds a minimal, self-contained project per library (also runnable by hand: cd tests/examples/cjson && mcpp run). validate.yml now:

• PR: runs only the example(s) for the packages whose descriptor changed (tests/run_example.sh);
• push / nightly / dispatch / scaffolding change / changed pkg without an example: runs the full legacy smoke regression.
• lint + mirror-cn-reachable always run over all descriptors.
• mcpp pin bumped 0.0.46 → 0.0.67.

Verification (local, real mcpp pipeline)

Both packages were built+run through real mcpp (fetch from the CN mirror → generate → compile → link → run):

• cjson ok=1 json={"answer":42,"lib":"cJSON"}
• nlohmann.json ok=true dump={"answer":42,"list":[1,2,3]}

Design doc

.agents/docs/2026-06-27-add-cjson-and-nlohmann-json-plan.md

[Sunrisepeak]

CI runs on the current release (0.0.67), incl. legacy smoke — root cause of an initial macOS/Windows failure

First CI run bumped mcpp 0.0.46 → 0.0.67 globally and the legacy smoke-macos/smoke-windows jobs failed with ld64.lld: error: undefined symbol: main.

Root cause: smoke_compat_{core,portable}.sh build gtest TEST() cases with no main() of their own, relying on gtest_main. The #168 fix gates gtest_main.cc behind the main feature; mcpp 0.0.67 honors the gating (excludes it by default → no main), while 0.0.46 ignored features and always linked it. So 0.0.46 was masking old-style gtest usage, not being "more correct".

Fix (not a version pin): declare the intended opt-in gtest = { version = "1.15.2", features = ["main"] }. Verified locally on 0.0.66 — full smoke_compat_core.sh runs (Running main() from gtest_main.cc, 2 tests PASSED). validate.yml is therefore unified on 0.0.67 for every job.

[Sunrisepeak]

• 增加对传统C/C++库, 以及基于make、cmake、xmake等工具项目的支持 - (含有不同方法解决方案) mcpp-community/mcpp#177