feat(macos): built-in default deployment floor 14.0 (rustc-style)

cargo's key portability win on macOS is that every Apple target carries
a built-in default deployment target (aarch64 = 11.0) instead of
floating with the host SDK — artifacts run on older systems with zero
configuration. Adopt the same: when neither MACOSX_DEPLOYMENT_TARGET
nor [build] macos_deployment_target is set and staticStdlib is on,
default the floor to 14.0 (the official LLVM static libc++ archives'
own floor). Mirrored in the fingerprint rule. Without staticStdlib the
toolchain/SDK default still applies (a dynamically-linked binary can't
promise a lower floor than the host libc++ anyway).

Author: Sunrisepeak

docs/05-mcpp-toml.md

@@ -98,8 +98,10 @@ macos_deployment_target = "14.0"   # macOS 产物的最低支持系统版本(仅
 (`LC_BUILD_VERSION minos`),即二进制能运行的最老 macOS。优先级与各生态
 惯例一致:环境变量 `MACOSX_DEPLOYMENT_TARGET`(单次调用的显式覆盖,
 cargo/rustc、cc 等同样尊重该变量)> 本字段(项目默认,类似 SwiftPM 的
-`platforms:`)> 工具链/SDK 默认。该值会进入 BMI 指纹——切换 target 会
-自动重建模块缓存。
+`platforms:`)> **内建默认 14.0**(rustc 式:target 自带默认 floor 而非
+跟随构建机 SDK;14.0 是工具链静态 libc++ 的下限,`static_stdlib = false`
+时回退工具链/SDK 默认)。该值会进入 BMI 指纹——切换 target 会自动重建
+模块缓存。
 
 C++ 标准不要通过 `build.cxxflags = ["-std=..."]` 配置。请使用:
 

src/build/flags.cppm

@@ -109,6 +109,18 @@ CompileFlags compute_flags(const BuildPlan& plan) {
     } else {
         macosDeploymentTarget = plan.manifest.buildConfig.macosDeploymentTarget;
     }
+    // Built-in default floor (rustc-style: every Apple target carries a
+    // default deployment target instead of floating with the host SDK).
+    // When neither the env var nor the manifest pins one, default to the
+    // floor of the toolchain's static libc++ archives (the official LLVM
+    // darwin archives are built for macOS 14) — artifacts are portable by
+    // default and don't silently change floor when the build host's SDK
+    // moves. Resolved here so the value also reaches the fingerprint via
+    // the same canonical-flags rule (see cli.cppm).
+    if (macosDeploymentTarget.empty() && mcpp::platform::is_macos
+        && plan.manifest.buildConfig.staticStdlib) {
+        macosDeploymentTarget = "14.0";
+    }
 
     f.cxxBinary = plan.toolchain.binaryPath;
     f.ccBinary = mcpp::toolchain::derive_c_compiler(plan.toolchain);

src/cli.cppm

@@ -600,6 +600,12 @@ std::string canonical_compile_flags(const mcpp::manifest::Manifest& m) {
         } else {
             dtv = m.buildConfig.macosDeploymentTarget;
         }
+        // Mirror flags.cppm's built-in default floor (14.0 with
+        // staticStdlib) so the fingerprint matches the flags actually
+        // emitted.
+        if (dtv.empty() && m.buildConfig.staticStdlib) {
+            dtv = "14.0";
+        }
         if (!dtv.empty()) {
             s += " macos_deployment_target=";
             s += dtv;