Framework internal-fault model (FEAT_0024)

Verification for the fail-fast boundary (Framework-invariant violati... (REQ_0123)), the user fatal handler (User-registered fatal handler (REQ_0125)), and the contained-item-panic guarantee (User-item panic is containe... (REQ_0124)).

Test Case: Framework panic routes to fatal handler TEST_0823
status: open
verifies: REQ_0123, REQ_0125
links incoming: REQ_0123, REQ_0125

In-process boundary test. A recording terminal (instead of aborting) is installed, and a synthetic non-item panic — panic!( "synthetic infra panic") — is injected through the boundary. The PoolWorker and InlineSubmit sites are driven end-to-end through a real threaded and inline Pool respectively; the ExecutorRunLoop site is verified at the guard_or_fatal mechanism level (a deterministic run-loop infra panic cannot be provoked without an artificial fault-injection seam). Asserts the fatal fired exactly once per site with the expected FatalSite and cause. Decouples the boundary check from any specific unreachable invariant violation.
Test Case: Default fail-fast aborts the process TEST_0824
status: open
verifies: REQ_0123
links incoming: REQ_0123

Subprocess test. A child process with the default (no-op) fatal handler triggers a framework-boundary panic; the parent asserts the child terminated via SIGABRT (abnormal, non-unwinding exit), not a clean exit and not a hang.
Test Case: Item panic is contained, not aborted TEST_0825
status: open
verifies: REQ_0124
links incoming: REQ_0124

Regression guard for the inner layer. A user item that panics in execute is caught and surfaced via on_app_error as a PanickedTask error, leaves the task in ``Running`` (containment is not a fault transition — Faulted is reserved for deadline breaches per Per-task overrun fault tran... (REQ_0070)), leaves independent sibling tasks running, and does not invoke the fatal handler or abort the process — proving the contained-panic path of User-item panic is containe... (REQ_0124) never escalates to the fail-fast path of Framework-invariant violati... (REQ_0123).