Bound FIFO inspection regressions and the generic CI test job #83

Problem

The project-inspection FIFO regressions can hang instead of fail:

• On main, test_fifo_configuration_is_never_opened relies on the production is_file() guard. Removing that guard blocks forever on a FIFO with no writer.
• PR Harden project compatibility inspection snapshots #79 adds nonblocking production opens and another FIFO-swap test, but removing O_NONBLOCK makes that test hang too.
• The generic GitHub Actions test job has no timeout-minutes, so a regression can consume the six-hour default.

Acceptance criteria

• Run each blocking-I/O regression behind a killable subprocess deadline; on timeout, terminate it and report the responsible test.
• Keep the assertions that production code never blocks on FIFOs or devices.
• Add a bounded timeout-minutes to the generic test job as a final backstop.
• Confirm the guarded tests finish promptly when the relevant production protection is deliberately removed.

This is distinct from #65: that issue is a PID-file setup race in descendant cleanup, while this issue bounds tests that may block indefinitely.