Add top-up / extend functions to lengthen or refund an active stream

[Omoboi-dev]

Today the only way to add funds to or lengthen a stream is to cancel and recreate it. Add first-class support.

What to do

• top_up(stream_id, amount) — sender deposits additional tokens into an active stream, recalculating rate_per_sec (or extending end_time, document which).
• extend(stream_id, new_end_time) — push out the end time, lowering the effective rate over the remaining deposit.

Requirements

• Sender-authed; only valid on Active streams.
• Must preserve already-earned/claimed amounts — a top-up or extend may never reduce what the recipient has already accrued.
• Checked integer math only (no overflow, no truncation that strands funds — see the dust handling in get_claimable).
• Emit events for the new state.

Acceptance criteria

• Unit tests covering claimable correctness immediately before and after top-up/extend.
• Tests that pre-top-up earnings are unchanged.
• cargo test passes.