Document CheckStatus contract and enforce CLI status-output examples in CI

[Copilot]

This follow-up codifies the CheckResult correctness contract (OK, ERROR, UNSUPPORTED) in project docs and adds explicit guardrails that CLI output continues to expose status-prefixed per-line results. The goal is to lock behavior before extending math feature scope.

• Status contract documentation

  • Added a dedicated CheckResult contract section in README.md defining:
    • OK: claim proved by the trusted internal checker
    • ERROR: checked and false, or expression invalid
    • UNSUPPORTED: outside current supported domain
  • Updated result wording to reflect structured status output rather than binary pass/fail phrasing.

• CLI output examples

  • Added explicit README examples showing current output shape for both success and failure:
    • Line N: OK
    • Line N: ERROR: ...

• Workflow assertions for output contract

  • Updated CI steps to assert emitted output matches required status line formats:
    • ^Line [0-9]+: OK$
    • ^Line [0-9]+: ERROR:
  • Adjusted expected-failure command handling to preserve and report the checker’s exit code while still validating stderr/stdout content patterns.

# CI assertions added for output contract
./build/pearlproof test1.proof | tee /tmp/test1.out
grep -Eq '^Line [0-9]+: OK$' /tmp/test1.out

./build/pearlproof test_errors.proof > /tmp/test_errors.out
status=$?
grep -Eq '^Line [0-9]+: ERROR: ' /tmp/test_errors.out