HWI parity: define error exit-status contract

[trevarj]

Goal

Define and enforce the process exit-status contract for HWI-compatible JSON errors.

Current state

The focused harness compares reference and candidate error JSON exactly but intentionally ignores process status because pinned Python HWI 3.2.0 sometimes exits with status 0 after returning an error object.

Scope

• Inventory exit statuses from pinned HWI 3.2.0 for every covered error class and command family.
• Decide whether BHWI should reproduce command-specific upstream statuses or expose one documented, consistent compatibility rule.
• Keep JSON error codes and messages unchanged unless the audit identifies a real mismatch.
• Update the CLI runner and focused harness to assert the chosen contract.
• Document any unavoidable divergence from HWI 3.2.0.

Acceptance criteria

• Reference exit behavior is recorded for the covered error matrix.
• BHWI has a documented exit-status contract for success, argument errors, unavailable actions, missing devices, and device failures.
• The parity harness asserts exit status instead of ignoring it.
• CLI unit/e2e tests cover representative status and JSON combinations.
• All focused parity runners and upstream final gates remain green.
• Required non-emulator checks pass.