v1.6.12

Mask quality preview metrics for foreground coverage, transparent coverage, transition area, edge density, jitter risk, and actionable warnings before export. Verification: python -m py_compile AlphaCut.py; pytest -q (90 passed); dynamic preview-worker smoke; native 900x650 Qt screenshot smoke; packaged AlphaCut-windows.exe --version and --runtime-info exit 0. Artifacts are locally built and checksummed; no code-signing certificate is configured on this machine. SHA256: AlphaCut-windows.exe 897bd36fdac91994e24ab6541c99b73b992689c52c7503124cd1f02a0c648461; AlphaCut-Setup-1.6.12.exe f457bdb9f05e649614e2be75e44e0d276182f5741b21910503c6467377261bef.