ci: add Windows LLVM smoke test + ci-fresh-install workflow

Main CI changes:
- ci-windows.yml: add explicit "Toolchain: LLVM — mcpp new → build → run"
  smoke test (previously only soft fallback via || true)
- All three main CIs (ci.yml, ci-macos.yml, ci-windows.yml) no longer
  have continue-on-error fresh user tests (removed in PR #62)

Toolchain coverage after this change:
  Linux:   gcc@16.1.0 ✓  musl-gcc@15.1.0 ✓  llvm@20.1.7 ✓
  macOS:   llvm@20.1.7 ✓ (comprehensive module tests)
  Windows: llvm@20.1.7 ✓ (explicit smoke test)

New workflow:
- ci-fresh-install.yml: tests released mcpp via xlings on clean machines
  (no caches). Validates real first-time user experience separately
  from PR code testing.

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -108,7 +108,16 @@ jobs:
           "$MCPP_SELF" --version
           echo ":: Self-host smoke PASS"
 
-      # Fresh user experience test moved to ci-fresh-install.yml (PR #63)
+      - name: "Toolchain: LLVM — mcpp new → build → run"
+        shell: bash
+        run: |
+          export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
+          "$MCPP_SELF" toolchain default llvm@20.1.7
+          TMP=$(mktemp -d)
+          cd "$TMP"
+          "$MCPP_SELF" new hello_llvm
+          cd hello_llvm
+          "$MCPP_SELF" run
 
       - name: Package Windows release zip
         id: package