EventSource: apply aligned reconnect jitter

[kdubb]

Summary

The cross-runtime EventSource alignment in #70 specifies downward jitter for exponential reconnect delays. Kotlin, Swift, and the new Python runtime apply a factor in the 0.9...1.0 range after the exact first retry, but the current TypeScript reconnect implementation remains deterministic.

Expected behavior

• Keep the first retry exactly equal to the current base retry interval.
• For escalated consecutive failed connections, calculate the capped exponential delay and apply downward jitter in the 0.9...1.0 range.
• Reset escalation after a connection opens, so a later clean close retries at the exact base interval.
• Add deterministic tests for the exact first retry, jitter bounds, cap behavior, and reset after open.

This is a follow-up to #70 and keeps TypeScript aligned with sunday-kt, sunday-swift, and sunday-python.