fix(build): report the staged file's absolute path and drop ninja's trailing pad

Two cosmetics found by driving a real staging failure end-to-end through ninja:
the `FAILED: <target>` line ninja emits carries a trailing space, and `$out` is
relative because ninja runs staging with cwd = the build directory — the reader
of that diagnostic is the person who has to go unlock the file.

Author: Sunrisepeak

src/build/ninja_backend.cppm

@@ -349,6 +349,9 @@ std::string filter_ninja_output(std::string_view output,
                 if (auto close = target.find(']'); close != std::string::npos)
                     target = ltrim_copy(target.substr(close + 1));
             }
+            while (!target.empty()
+                   && std::isspace(static_cast<unsigned char>(target.back())))
+                target.pop_back();
             if (target.empty()) continue;
             filtered += "failed: ";
             filtered += target;

src/build/stage.cppm

@@ -124,6 +124,11 @@ std::error_code write_once(const std::filesystem::path& src,
 std::string failure_message(const std::filesystem::path& src,
                             const std::filesystem::path& dst,
                             const std::error_code& ec) {
+    // ninja runs staging with cwd = the build directory, so `$out` is relative.
+    // Print it absolute: the reader has to go find (or unlock) this file.
+    std::error_code aec;
+    auto shown = std::filesystem::absolute(dst, aec);
+    if (aec) shown = dst;
     return std::format(
         "cannot stage file into the build directory\n"
         "  file:  {}\n"
@@ -135,7 +140,7 @@ std::string failure_message(const std::filesystem::path& src,
         "      editor/IDE indexer, antivirus, or — for a .dll — a still-running\n"
         "      program from a previous `mcpp run`. Close it or restart clangd,\n"
         "      then re-run the build.",
-        dst.string(), src.string(), ec.value(), ec.message());
+        shown.string(), src.string(), ec.value(), ec.message());
 }
 
 } // namespace

tests/unit/test_ninja_backend.cpp

@@ -876,7 +876,7 @@ TEST(NinjaBackend, FilterKeepsStagingDiagnosticsAndFailedTarget) {
     std::string raw =
         "ninja: Entering directory `target/x86_64-linux-gnu/fp'\n"
         "[1/3] STAGE pcm.cache/std.pcm\n"
-        "FAILED: [code=1] pcm.cache/std.pcm\n"
+        "FAILED: [code=1] pcm.cache/std.pcm \n"
         "/opt/mcpp/bin/mcpp stage --output pcm.cache/std.pcm /cache/std.pcm\n"
         "error: cannot stage file into the build directory\n"
         "hint: another process has this file memory-mapped, loaded or open.\n"
@@ -885,7 +885,9 @@ TEST(NinjaBackend, FilterKeepsStagingDiagnosticsAndFailedTarget) {
     auto filtered = filter_ninja_output(raw, prefixes);
 
     // Which output failed is now preserved (it used to be dropped entirely).
-    EXPECT_NE(filtered.find("failed: pcm.cache/std.pcm"), std::string::npos) << filtered;
+    // ninja pads the target list with a trailing space; the normalized line
+    // must not inherit it.
+    EXPECT_NE(filtered.find("failed: pcm.cache/std.pcm\n"), std::string::npos) << filtered;
     EXPECT_EQ(filtered.find("[code=1]"), std::string::npos) << filtered;
     // The diagnostic survives; the echoed command line does not.
     EXPECT_NE(filtered.find("error: cannot stage file"), std::string::npos) << filtered;