fix: preserve SUUS event timestamps

[very-code-com]

Summary

• parse fractional seconds and timezone suffixes returned in SUUS xsd:time values
• return null for missing or invalid event dates instead of substituting the current time
• document the nullable StatusEvent occurredAt contract for 1.1.0
• cover valid, invalid, and repeated-poll timestamp behavior

Verification

• PHPUnit unit suite: 387 tests, 615 assertions
• PHPStan level 8: no errors
• PHP lint: src, tests, and examples
• SUUS sandbox integration suite: 5 tests, 26 assertions
• SUUS sandbox getEvents stability: 1 test, 7 assertions
• live J_CR check: raw 12:18:03.04 parsed as 12:18:03.040000 and stable across polls