feat: add compat.fmt 12.2.0 (C++ source compat)

fmt is built as a C++-source compat lib (same shape as compat.gtest):
compile src/format.cc + src/os.cc into the `fmt` lib and expose include/
for `#include <fmt/format.h>`. src/fmt.cc (C++20 module unit) is excluded;
src/fmt-c.cc is source-gated behind an optional `c-api` feature.

Verified locally on mcpp 0.0.81 (== CI): `mcpp test -p fmt` builds
compat.fmt from source and the behavioral assertions pass. No mcpp-res
write access, so the download url is a plain upstream string (CN mirror to
be added by a maintainer).

Co-Authored-By: Claude <noreply@anthropic.com>

Author: pyc-copilot

mcpp.toml

@@ -13,6 +13,7 @@ members = [
     "tests/examples/cjson",
     "tests/examples/core",
     "tests/examples/eigen",
+    "tests/examples/fmt",
     "tests/examples/gui-stack",
     "tests/examples/imgui",
     "tests/examples/imgui-window",

pkgs/c/compat.fmt.lua

@@ -0,0 +1,65 @@
+-- Form B inline descriptor for {fmt} — a modern formatting library for C++.
+-- C++-source compat build (same shape as compat.gtest): compile the library's
+-- non-header translation units into a lib and expose the `include/` tree via
+-- include_dirs, so consumers write `#include <fmt/core.h>` / `<fmt/format.h>`.
+--
+-- Sources: fmt's compiled implementation is `src/format.cc` (core formatting)
+-- and `src/os.cc` (optional OS-specific I/O — fmt::ostream, file descriptors).
+-- These two are exactly upstream CMake's `fmt` target sources. NOT globbed:
+--   * `src/fmt.cc`   — the C++20 named-module unit (starts with `module;`); it
+--                      must be compiled as a module interface, not a plain TU,
+--                      so it is deliberately excluded here.
+--   * `src/fmt-c.cc` — the optional C API; gated behind the `c-api` feature
+--                      below (excluded by default, same pattern as compat.gtest
+--                      gating gtest_main behind `main`).
+--
+-- All `mcpp` paths are GLOBS relative to the verdir; the leading `*/` absorbs
+-- the GitHub tarball's `fmt-<tag>/` wrap layer.
+package = {
+    spec        = "1",
+    namespace   = "compat",
+    name        = "compat.fmt",
+    description = "A modern formatting library for C++",
+    licenses    = {"MIT"},
+    repo        = "https://github.com/fmtlib/fmt",
+    type        = "package",
+
+    xpm = {
+        linux = {
+            ["12.2.0"] = {
+                url    = "https://github.com/fmtlib/fmt/archive/refs/tags/12.2.0.tar.gz",
+                sha256 = "8b852bb5aa6e7d8564f9e81394055395dd1d1936d38dfd3a17792a02bebd7af0",
+            },
+        },
+        macosx = {
+            ["12.2.0"] = {
+                url    = "https://github.com/fmtlib/fmt/archive/refs/tags/12.2.0.tar.gz",
+                sha256 = "8b852bb5aa6e7d8564f9e81394055395dd1d1936d38dfd3a17792a02bebd7af0",
+            },
+        },
+        windows = {
+            ["12.2.0"] = {
+                url    = "https://github.com/fmtlib/fmt/archive/refs/tags/12.2.0.tar.gz",
+                sha256 = "8b852bb5aa6e7d8564f9e81394055395dd1d1936d38dfd3a17792a02bebd7af0",
+            },
+        },
+    },
+
+    mcpp = {
+        language     = "c++23",
+        import_std   = false,
+        include_dirs = { "*/include" },
+        sources      = { "*/src/format.cc", "*/src/os.cc" },
+        targets      = { ["fmt"] = { kind = "lib" } },
+        -- Optional C API (src/fmt-c.cc, <fmt/fmt-c.h>), source-gated like
+        -- compat.gtest's `main` / compat.cjson's `utils`. NOTE: mcpp 0.0.81 does
+        -- not yet propagate a feature requested on a DEPENDENCY into its compile
+        -- plan, so this object is not built and fmt_vformat is unresolved even
+        -- with the feature on — the same gap compat.eigen notes for `eigen_blas`.
+        -- Kept form-correct; lights up once mcpp propagates dependency features.
+        features     = {
+            ["c-api"] = { sources = { "*/src/fmt-c.cc" } },
+        },
+        deps         = { },
+    },
+}

tests/examples/fmt/mcpp.toml

@@ -0,0 +1,12 @@
+# {fmt} test project: depends on compat.fmt (built from source via this repo's
+# own index) and asserts behavior under `mcpp test`. Part of the mcpp-index
+# self-referential workspace.
+[package]
+name = "fmt-tests"
+version = "0.1.0"
+
+[indices]
+compat = { path = "../../.." }
+
+[dependencies.compat]
+fmt = "12.2.0"

tests/examples/fmt/tests/format_test.cpp

@@ -0,0 +1,19 @@
+// Behavioral test: exercise {fmt}'s compiled implementation (src/format.cc via
+// the fmt lib target) — not just header inlines — and assert the results.
+// Returns non-zero on any mismatch.
+#include <fmt/format.h>
+#include <cmath>
+#include <string>
+
+int main() {
+    std::string a = fmt::format("{} + {} = {}", 2, 3, 2 + 3);
+    std::string b = fmt::format("{:08.3f}", 3.14159);
+    std::string c = fmt::format("{0}-{1}-{0}", "x", "y");
+    std::string d = fmt::format("{:#x}", 255);
+
+    bool ok = a == "2 + 3 = 5"
+              && b == "0003.142"
+              && c == "x-y-x"
+              && d == "0xff";
+    return ok ? 0 : 1;
+}