Add event deduplication via optional id parameter

[hownowstephen]

Summary

• Add optional id: and timestamp: keyword arguments to track and track_anonymous for event deduplication and explicit timestamp control
• The id should be a ULID — no client-side validation, API rejects invalid values
• Matches the Go client's WithEventID() pattern

Fixes #107. Replaces #108 and #85.

⚠️ Breaking change

Adding keyword arguments to track and track_anonymous means callers using implicit hash syntax must add explicit braces:

# Before (no longer works — Ruby interprets bare key:value as keyword args)
client.track(5, "purchase", type: "socks", price: "13.99")

# After
client.track(5, "purchase", { type: "socks", price: "13.99" })

This is a semver-major change. All README examples updated.

Docs team: The Track API reference Ruby tab and any other docs showing track()/track_anonymous() Ruby examples should be checked for implicit hash syntax.

Test plan

• CI passes on Ruby 3.3, 3.4, 4.0, head
• id field present in request body when provided
• id field absent when not provided
• timestamp keyword arg sets top-level timestamp
• timestamp in attributes hash still works (backwards compat for that path)
• Invalid timestamps (milliseconds, strings, dates) omitted from body

[cursor]

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