Fix event iterator start time clamping

[preko-p]

Summary

• Clamp Event#iterator(startTime) so an earlier caller-provided start time cannot move recurrence expansion before the event DTSTART.
• Keep later startTime behavior intact.
• Repair the existing iterator test typo and add earlier-start regression coverage.

Fixes #456.

Verification

• PATH=/opt/homebrew/bin:$PATH npm run test-unit -- test/event_test.js (62 passing in no-space verification clone)
• PATH=/opt/homebrew/bin:$PATH npm run lint
• PATH=/opt/homebrew/bin:$PATH npm run test (1021 passing, 1 pending, plus 5 acceptance tests in no-space verification clone)
• git diff --check

Note: the original local checkout path contains a space, which trips the existing test helper's URL.pathname handling before Mocha tests run. I verified the exact same diff in a temporary no-space clone.

AI assistance

Prepared with AI assistance and manually verified before submission.