[JacobStephens2]

Summary

 test fixture executable
-  fs::write / fs::copy in a parallel test process
+  shared write_executable / copy_executable in a child process

Closes #1110

Test seams

• Existing CLI, printed installer, PATH fake, Git hook, and executable replacement tests.
• Concurrent repetitions of the complete console_install integration test binary.

Evidence

• Before: 50 repetitions of cargo test --test console_install, four
  concurrent: 46 passed, 4 failed with ExecutableFileBusy: Text file busy
  (repetitions 3, 6, 31 and 46; 22.92 seconds).
  After: the same 50 repetitions at e4923c7 all passed (60.80
  seconds while the full suites were also running). The first implementation
  also passed 50/50 in 28.45 seconds.
• cargo clippy --all-targets -- -D warnings and cargo fmt --check passed on
  final HEAD bac9e7e.
• macOS CI caught an import used only by Linux setup tests; bac9e7e gives the
  import the same Linux guard. The updated macOS gate passed. Final-commit Linux
  Rust, site, Console and release build checks also passed in
  GitHub checks.
• /usr/bin/time -p scripts/affected-tests main passed: 2,417 Rust tests passed,
  56 skipped. Wall time: 2,142.21 seconds, including compilation and build-lock
  waits while other validation was running.
• /usr/bin/time -p cargo test passed. Wall time: 2,188.79 seconds, including
  compilation (23.17 seconds) and build-lock waits. Both local full-suite runs
  overlapped on this shared host.

Review coverage

Independent Standards and Spec reviews against main read all 19 changed files,
including fresh final reviews through bac9e7e. Standards' one initial optional
Duplicated Code finding was addressed by sharing executable copies. Both final
reviews reported no findings.

Changed files left unread: none, on either axis. Generated-output exclusions:
none. Final reports are retained with the Session trace.

Unaddressed findings

Standards

None. The sole finding was addressed in e4923c7.

Spec

None; no findings were reported.

Security

None. Focused review of the 3016eda...HEAD diff (test-only: test-support write_executable/copy_executable plus call-site conversions) found no introduced trust-boundary violation. The helper passes the path as a positional shell parameter inside double quotes and the contents over stdin, and copies via cp with no shell, so test-controlled paths and contents cannot inject shell commands; all changed sources are #[cfg(test)] or integration tests, so no production behavior changed. No proof-of-concept test was warranted and none was added.

Security review outcome

No unaddressed introduced findings.

Merge Danger

Door: two-way

Blast Radius: tests

Test fixtures and shared test support. Reverting restores the executable-write
race.

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