fix: CI — tee to file instead of /dev/stderr (Windows compat), precise grep

- Replace `tee /dev/stderr` with `tee build.log` (Git Bash on Windows
  doesn't have /dev/stderr)
- All grep assertions read from build.log file
- musl grep: "musl" → "Resolved gcc@15.1.0-musl" (more precise)

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -118,7 +118,7 @@ jobs:
           MCPP=/tmp/mcpp-fresh.exe
           "$MCPP" toolchain default llvm@20.1.7
           "$MCPP" clean --bmi-cache
-          "$MCPP" build 2>&1 | tee /dev/stderr | grep -q "Resolved llvm@20.1.7"
+          "$MCPP" build 2>&1 | tee build.log; grep -q "Resolved llvm@20.1.7" build.log
 
       - name: Package Windows release zip
         id: package

.github/workflows/ci.yml

@@ -137,20 +137,20 @@ jobs:
       - name: "Toolchain: GCC — build mcpp + test"
         run: |
           "$MCPP" clean
-          "$MCPP" build 2>&1 | tee /dev/stderr | grep -q "Resolved gcc@16.1.0"
+          "$MCPP" build 2>&1 | tee build.log; grep -q "Resolved gcc@16.1.0" build.log
           "$MCPP" test
 
       - name: "Toolchain: musl-gcc — build mcpp (--target)"
         run: |
           "$MCPP" clean
-          "$MCPP" build --target x86_64-linux-musl 2>&1 | tee /dev/stderr | grep -q "musl"
+          "$MCPP" build --target x86_64-linux-musl 2>&1 | tee build.log; grep -q "Resolved gcc@15.1.0-musl" build.log
 
       - name: "Toolchain: LLVM — build mcpp"
         run: |
           "$MCPP" toolchain install llvm 20.1.7
           # Override project toolchain to use LLVM for this build
           sed -i 's/^default = "gcc@16.1.0"/default = "llvm@20.1.7"/' mcpp.toml
           "$MCPP" clean
-          "$MCPP" build 2>&1 | tee /dev/stderr | grep -q "Resolved llvm@20.1.7"
+          "$MCPP" build 2>&1 | tee build.log; grep -q "Resolved llvm@20.1.7" build.log
           # Restore
           sed -i 's/^default = "llvm@20.1.7"/default = "gcc@16.1.0"/' mcpp.toml