ci: each platform's every toolchain builds mcpp itself

Merge redundant self-host smoke steps into unified toolchain steps.
Each supported toolchain per platform does: clean → build mcpp → verify.

  Linux:   gcc@16.1.0 (+ test), musl-gcc@15.1.0, llvm@20.1.7
  macOS:   llvm@20.1.7
  Windows: llvm@20.1.7

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -306,14 +306,12 @@ jobs:
           "$MCPP" toolchain default llvm@20.1.7
           bash tests/e2e/run_all.sh
 
-      - name: Self-host smoke (freshly-built mcpp builds itself again)
+      - name: "Toolchain: LLVM — build mcpp (self-host)"
         run: |
           MCPP=$(find target -path "*/bin/mcpp" | head -1)
           MCPP=$(cd "$(dirname "$MCPP")" && pwd)/$(basename "$MCPP")
           test -x "$MCPP"
-          export PATH="$(dirname "$MCPP"):$PATH"
+          "$MCPP" toolchain default llvm@20.1.7
+          "$MCPP" clean
           "$MCPP" build
           "$MCPP" --version
-          echo ":: Self-host smoke PASS"
-
-      # Fresh user experience test moved to ci-fresh-install.yml (PR #63)

.github/workflows/ci-windows.yml

@@ -100,21 +100,14 @@ jobs:
           "$MCPP_SELF" toolchain default llvm@20.1.7 2>/dev/null || true
           bash tests/e2e/run_all.sh
 
-      - name: Self-host smoke (freshly-built mcpp builds itself again)
-        shell: bash
-        run: |
-          export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
-          "$MCPP_SELF" build
-          "$MCPP_SELF" --version
-          echo ":: Self-host smoke PASS"
-
-      - name: "Toolchain: LLVM — build mcpp"
+      - name: "Toolchain: LLVM — build mcpp (self-host)"
         shell: bash
         run: |
           export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
           "$MCPP_SELF" toolchain default llvm@20.1.7
           "$MCPP_SELF" clean
           "$MCPP_SELF" build
+          "$MCPP_SELF" --version
 
       - name: Package Windows release zip
         id: package

.github/workflows/ci.yml

@@ -128,18 +128,13 @@ jobs:
           "$MCPP" toolchain default gcc@16.1.0
           bash tests/e2e/run_all.sh
 
-      - name: Self-host smoke (freshly-built mcpp builds itself again)
-        run: |
-          MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)")
-          "$MCPP" build
-          "$MCPP" test
-
-      - name: "Toolchain: GCC — build mcpp"
+      - name: "Toolchain: GCC — build mcpp + test"
         run: |
           MCPP=$(realpath "$(find target -type f -name mcpp -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2)")
           "$MCPP" toolchain default gcc@16.1.0
           "$MCPP" clean
           "$MCPP" build
+          "$MCPP" test
 
       - name: "Toolchain: musl-gcc — build mcpp"
         run: |