ci: toolchain smoke tests build mcpp itself, not just hello world

Each toolchain smoke test now builds mcpp from source (self-host)
instead of a trivial hello-world project. This validates that the
toolchain can compile a real C++23 modules project.

Coverage:
  Linux:   gcc@16.1.0 builds mcpp ✓
           musl-gcc@15.1.0 builds mcpp ✓
           llvm@20.1.7 builds mcpp ✓
  macOS:   llvm@20.1.7 builds mcpp ✓ (default, in self-host smoke)
  Windows: llvm@20.1.7 builds mcpp ✓ (new explicit step)

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -108,7 +108,13 @@ jobs:
           "$MCPP_SELF" --version
           echo ":: Self-host smoke PASS"
 
-      # Fresh user experience test moved to ci-fresh-install.yml (PR #63)
+      - name: "Toolchain: LLVM — build mcpp"
+        shell: bash
+        run: |
+          export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
+          "$MCPP_SELF" toolchain default llvm@20.1.7
+          "$MCPP_SELF" clean
+          "$MCPP_SELF" build
 
       - name: Package Windows release zip
         id: package

.github/workflows/ci.yml

@@ -134,34 +134,24 @@ jobs:
           "$MCPP" build
           "$MCPP" test
 
-      - name: "Toolchain: GCC — mcpp new → build → run"
+      - name: "Toolchain: GCC — build mcpp"
         run: |
           MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)")
           "$MCPP" toolchain default gcc@16.1.0
-          TMP=$(mktemp -d)
-          cd "$TMP"
-          "$MCPP" new hello_gcc
-          cd hello_gcc
-          "$MCPP" run
+          "$MCPP" clean
+          "$MCPP" build
 
-      - name: "Toolchain: musl-gcc — mcpp new → build → run"
+      - name: "Toolchain: musl-gcc — build mcpp"
         run: |
           MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)")
-          "$MCPP" toolchain install gcc 15.1.0-musl
           "$MCPP" toolchain default gcc@15.1.0-musl
-          TMP=$(mktemp -d)
-          cd "$TMP"
-          "$MCPP" new hello_musl
-          cd hello_musl
-          "$MCPP" run
+          "$MCPP" clean
+          "$MCPP" build
 
-      - name: "Toolchain: LLVM — install + mcpp new → build → run"
+      - name: "Toolchain: LLVM — build mcpp"
         run: |
           MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)")
           "$MCPP" toolchain install llvm 20.1.7
           "$MCPP" toolchain default llvm@20.1.7
-          TMP=$(mktemp -d)
-          cd "$TMP"
-          "$MCPP" new hello_llvm
-          cd hello_llvm
-          "$MCPP" run
+          "$MCPP" clean
+          "$MCPP" build