Define stable JSON event protocol

Closes #10

Summary

• add a typed, versioned event envelope for run, suite, test, hook, output, retry, and coverage data
• share standard JSON serialization across the JSON reporter, SSE UI, and test history
• add escaping, per-event contract, reporter, UI transport, and history persistence tests
• document the v1 schema and compatibility policy

Verification

• zig fmt --check .
• zig build test
• zig build examples
• zig build -Dtarget=x86_64-windows