Run the integration suite on Windows

[lofoneh]

Problem

write_fake_pgbot generates the fake pgbot as a POSIX shell script, so the
three integration test files are #![cfg(unix)]. On Windows they don't fail —
they compile away:

                      before   after
  src/ unit tests         96      96
  cli_add                  0      12
  monitor                  0       2
  runner_integration       0       8

22 tests, including dsn_travels_by_env_not_argv and
add_failure_saves_nothing_and_leaks_nothing.

Change

tests/bin/fake_pgbot.rs ports the fixture to Rust as a fake-pgbot bin
target. write_fake_pgbot copies it into the test's temp dir, where it finds
invocations.log / running.<pid> / peaks.log via current_exe().parent() —
so the helper's signature is unchanged and no assertion was touched. Drops the
three #![cfg(unix)] lines, adds windows-latest to CI. saved_file_is_0600
stays unix-only.

Also fixes a latent flake: writeln! emits the value and the newline as
separate writes, so concurrent fakes interleaved into peaks.log as
"22\n\n1\n" and the cap test read a peak of 22. Now one write_all.

fmt, clippy and cargo test --locked pass on Windows; linux and macOS
unverified until CI runs.