Add frailbox self-test JSON summary

[h1065153539-create]

Adds a lightweight C self-test summary harness for frailbox.

What changed:

• Adds frailbox/tests/selftest_summary.c to emit the self-test result summary.
• Preserves the default text path with make -C frailbox selftest.
• Adds JSON output via make -s -C frailbox selftest-json.
• Adds success/failure fixture validation via make -s -C frailbox selftest-json-fixtures.
• Documents the JSON summary in docs/OPERATIONS.md.

Validation performed:

• make -C frailbox selftest-json-fixtures
• make -s -C frailbox selftest-json
• Parsed JSON output locally.
• Confirmed failure fixture exits non-zero and includes failure_reason.

Diagnostic bundle:

• diagnostic/build-137421ad.logd
• diagnostic/build-137421ad.json

Closes #5.

[Soengkit]

Closing this PR because the linked fork issue is only a closed payout tracker for an already submitted upstream PR. This fork is not an active bounty intake or payment authority, and it is not accepting external submissions or payment details here.