Reject far-future stream start times

[code-0-stella]

Reject far-future stream start times

Summary

Rejects stream creation when start_time is too far in the future, preventing the protocol from escrowing a full future deposit for a stream that will not begin for an extended period.

This closes the gap where create_stream already rejected start_time < now, but still accepted far-future schedules. That allowed a sender to lock funds immediately with no meaningful accrual and no timely early exit beyond a manual cancel.

What changed

• Added an upper bound on start_time - now based on the governor’s configured maximum scheduling horizon.
• Rejected values beyond that ceiling with StartTimeTooFarInFuture.
• Added regression tests covering:
  • far-future open-ended streams
  • far-future fixed-duration streams
  • edge case at the ceiling
  • just-past-the-ceiling reject path

Why

A stream scheduled years in the future still escrows the full deposit immediately. That leaves funds idle, increases recipient index bloat risk, and creates a poor UX trap for users who expect the stream to begin promptly.

Verification

Verified with focused regression tests covering the far-future start-time cases on the fresh task branch.

Closes #395

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