ci(coin-matrix): gate on src/c2pool/main_<coin>.cpp entrypoint (#66)

The presence check keyed on src/impl/<coin>/main_<coin>.cpp, but the
per-coin entrypoint actually lives at src/c2pool/main_<coin>.cpp (the
convention documented in the file header comment). The btc binary
entrypoint is src/c2pool/main_btc.cpp with no src/impl/btc/main_btc.cpp,
so the btc matrix job silently skipped as a false green. Gate now
matches the real entrypoint path while retaining the src/impl/<coin>/
directory presence check.

Author: frstrtr

.github/workflows/coin-matrix.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Check source presence
         id: presence
         run: |
-          if [ -f "src/impl/${{ matrix.coin }}/main_${{ matrix.coin }}.cpp" ] && [ -d "src/impl/${{ matrix.coin }}" ]; then
+          if [ -f "src/c2pool/main_${{ matrix.coin }}.cpp" ] && [ -d "src/impl/${{ matrix.coin }}" ]; then
             echo "exists=1" >> "$GITHUB_OUTPUT"
             echo "::notice::c2pool-${{ matrix.coin }} source present — building."
           else