fix: keep built binary after app shutdown (#911)

Air could delete build.entrypoint while stopping the previous app process when stop_on_error was enabled. After builds were changed to keep the old app alive until a replacement build succeeds, that cleanup could remove the freshly rebuilt binary before it was started, causing issue #910.

Failed builds should not delete the last executable either. Keeping the binary in place makes stop_on_error stop the process without destroying the artifact a later rebuild or manual restart may need.

Windows needs different sequencing because running executables are locked. Stop the old process before build.cmd on Windows, while keeping the retained-app behavior on Unix platforms.

Validation: go test ./runner -run 'TestAddPlatformOverridesForInit|TestPlatformBuildOverridesSelection|TestShouldStopBinBeforeBuild|TestStopBinBeforeBuildIfNeeded|TestBuildRunKeepsIssue910GoBuildEntrypoint' -count=1 -v; go test ./runner -run 'TestBuildRunKeepsIssue910GoBuildEntrypoint|TestShouldStopBinBeforeBuild|TestBuildRunKeeps.*Binary.*StopOnError|TestBuildRunStopsExistingBinWhenBuildFailsWithStopOnError|TestBuildRunStopsExistingBinAfterSuccessfulBuild|TestRebuild$' -count=1 -v; go test ./...; make check

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Co-authored-by: Marius van Niekerk <mariusvniekerk@mbp-marius-kenn.emperor-gopher.ts.net>
Co-authored-by: OpenAI Codex <noreply@openai.com>