docs(design): review decisions, three measured corrections, and the task list

The four review questions are answered: windows_subsystem takes "windows",
windows_entry ships now, a resolve-time refusal is enough for #613, and the
graph-wide switch is an abi table.

Measuring the plan corrected it three times. The #613 refusal already exists as
requires = ["mcpp:c++-abi=libstdc++"]. The whole-graph channel G2 asked for is
dialect_cxxflags: -pthread there links the openkal-emscripten threads feature,
while the same flag in cxxflags reproduces the recorded error. And a new
defect, T1: mcpp build --toolchain llvm@22.1.8 after a gcc build replays the
gcc build in 0.00s.

Author: speak-agent