fix: the toolchain stamp formats its numbers explicitly, and the design record carries front matter

`std::to_string` over a filesystem clock's rep and over `uintmax_t` is
ambiguous on libc++, so the three sites that write or compare the stamp of a
toolchain named by path format the two values through `std::format` with an
explicit type. The design record gains the `subject`/`status` front matter
every record dated 2026-09-08 or later carries.

Author: speak-agent

.agents/docs/2026-10-01-tool-and-toolchain-sources-design.md

@@ -1,3 +1,8 @@
+---
+subject: design
+status: landed
+---
+
 # 工具与工具链的来源：声明、编程决定、可观察
 
 日期：2026-10-01。状态：已落地（mcpp 2026.10.1.3,mcpp#755)。

.agents/docs/README.md

@@ -30,6 +30,7 @@ Records that declare one. Everything else is listed by date below.
 
 ### design
 
+- [工具与工具链的来源：声明、编程决定、可观察](2026-10-01-tool-and-toolchain-sources-design.md) — landed
 - [A pack's build reported as a build, and a unit's compile independent of the member selection: triage and design (#753, #751)](2026-10-01-pack-drive-and-selection-independent-compile-design.md) — landed
 - [Member selection, build programs prepared once, a pack over several members, and the output streams of `mcpp run`: the plan for the release after 2026.9.30.2 (#748, #749, #750)](2026-09-30-member-selection-and-build-program-cost-plan.md) — landed
 - [The build's wall time, its progress count, a hang after the build, and #732 and #744: measurements and a remediation plan](2026-09-30-build-wall-time-progress-count-and-hang-plan.md) — landed
@@ -113,7 +114,7 @@ Records that declare one. Everything else is listed by date below.
 
 ### 2026-10
 
-- [工具与工具链的来源：声明、编程决定、可观察](2026-10-01-tool-and-toolchain-sources-design.md)
+- [工具与工具链的来源：声明、编程决定、可观察](2026-10-01-tool-and-toolchain-sources-design.md) — landed
 - [A pack's build reported as a build, and a unit's compile independent of the member selection: triage and design (#753, #751)](2026-10-01-pack-drive-and-selection-independent-compile-design.md) — landed
 ### 2026-09
 

src/build/execute.cppm

@@ -1572,8 +1572,12 @@ bool local_toolchain_unchanged(const std::filesystem::path& outputDir) {
         if (ec) return false;
         const auto time = std::filesystem::last_write_time(p, ec);
         if (ec) return false;
-        if (std::to_string(size) != line.substr(0, t1)
-            || std::to_string(time.time_since_epoch().count()) != line.substr(t1 + 1, t2 - t1 - 1))
+        // `std::format`, not `std::to_string`: the clock's rep and
+        // `uintmax_t` both convert to two integer overloads of the latter, and
+        // libc++ calls that ambiguous.
+        if (std::format("{}", static_cast<std::uint64_t>(size)) != line.substr(0, t1)
+            || std::format("{}", static_cast<std::int64_t>(time.time_since_epoch().count()))
+                   != line.substr(t1 + 1, t2 - t1 - 1))
             return false;
     }
     return true;

src/build/prepare/local_toolchain.cpp

@@ -100,9 +100,9 @@ std::string local_identity(const mcpp::toolchain::Toolchain& tc) {
     auto stamp = [&](const fs::path& p) {
         std::error_code ec;
         mix(p.generic_string());
-        mix(std::to_string(fs::file_size(p, ec)));
+        mix(std::format("{}", static_cast<std::uint64_t>(fs::file_size(p, ec))));
         auto t = fs::last_write_time(p, ec);
-        mix(std::to_string(t.time_since_epoch().count()));
+        mix(std::format("{}", static_cast<std::int64_t>(t.time_since_epoch().count())));
     };
     stamp(tc.binaryPath);
     for (auto const& [role, p] : tc.toolOverrides) { mix(role); stamp(p); }

src/build/prepare/records.cpp

@@ -598,7 +598,10 @@ void step13_resolution_json(PrepareState& state, BuildContext& ctx) {
                     std::error_code fe;
                     const auto size = std::filesystem::file_size(p, fe);
                     const auto time = std::filesystem::last_write_time(p, fe);
-                    st << size << '\t' << time.time_since_epoch().count() << '\t'
+                    // The same two spellings the fast path compares against.
+                    st << std::format("{}", static_cast<std::uint64_t>(size)) << '\t'
+                       << std::format("{}", static_cast<std::int64_t>(
+                              time.time_since_epoch().count())) << '\t'
                        << p.string() << '\n';
                 };
                 put(ctx.tc.binaryPath);