Add --maxfail option to stop after N failures

[renaudcepre]

Feature Request

Add a --maxfail=N option (similar to pytest) to stop test execution after N failures/errors.

Current behavior

• -x / --exitfirst: stops after the first failure

Proposed behavior

• --maxfail=N: stops after N failures/errors
• -x would be equivalent to --maxfail=1

Use cases

• CI pipelines: fail fast but allow a few flaky tests
• Large test suites: don't wait for 1000 failures when 10 is enough to know something is broken
• Memory optimization: fewer failures stored in reporter lists

Implementation notes

• Modify ParallelExecutor to check failure count against threshold
• Add CLI argument in protest/cli/main.py
• Update _ParallelExecutionState to track failure count (already done with counts)

Priority

Low - nice to have