Fix fatal startup exit status

[snapetech]

What changed

• invoke the injected exit action with status 1 when startup catches an unexpected fatal exception
• add a focused regression for the generic termination handler
• document the startup exit-code gotcha and update project tracking notes

Why

An occupied HTTP port produced a fatal Kestrel startup exception, but the catch path only logged it and returned. The process therefore exited with status 0, causing service managers and deployment probes to report a failed start as successful.

Validation

• focused StartupWebApplicationRunnerTests: passed
• live occupied-HTTP-port process check: exited 1
• repository lint: passed
• local identity and GitHub target gates: passed
• full unit run: 4,352 passed, 1 unrelated existing failure in PublicProtocolAnonymousActionTests.StreamingController_RemainsAnonymousForTokenBackedTransport because current main exposes both CreateShareTicket and Get while the assertion expects only Get

No release tag was created.