fix: implement partial_cancel entry point on streaming contract

[christabel888]

Problem

partial_cancel was listed in the README's contract function table ("Sender reduces a stream's locked balance, releasing funds back") but was never actually implemented in impl StreamingContract — there was no entry point, and no PartialCancelEvent type either. Prior test coverage that referenced it had already been removed from test_features.rs in earlier merged PRs, leaving the feature entirely missing.

Fix

Implements partial_cancel(stream_id, amount) in contracts/streaming/src/lib.rs, following the existing cancel/top_up patterns:

• Only the stream's sender may call it (require_auth), and only while the contract is unpaused.
• Rejects an already-cancelled or already-ended stream (StreamCancelled / StreamEnded).
• Computes the currently unlocked vs. locked balance via the existing unlocked_amount helper; amount must be > 0 and <= locked (InvalidAmount otherwise).
• Transfers amount back to the sender and reduces deposited_amount by it — the stream is not terminated, unlike cancel.
• Re-anchors the vesting schedule at the current moment (the mirror image of what top_up already does when funds are added): whatever is unlocked as of now is frozen as the new "cliff", and the reduced remaining balance streams linearly to end_time at a recomputed rate (RateIsZero if that rate would round to zero).
• Adds a new PartialCancelEvent (stream_id, sender, recipient, amount_refunded, new_deposited_amount, timestamp), published on success.

Also adds test coverage in contracts/streaming/src/test_features.rs:

• test_partial_cancel_returns_locked_funds_and_keeps_stream_active — happy path, confirms the stream stays active, deposited amount drops by the refund, already-unlocked funds are untouched, and the stream keeps streaming afterward.
• test_partial_cancel_rejects_amount_exceeding_locked_balance — over-withdrawal guard.
• test_partial_cancel_rejects_already_cancelled_stream — guard against calling on a fully-cancelled stream.

Notes / TODOs

• Not run through cargo test/cargo build in this environment — implementation and tests were written directly from reading cancel/top_up and the existing test harness conventions in test_features.rs. Please run the test suite in CI/locally before merge.
• docs/api-reference.md does not yet have a partial_cancel section (unlike cancel/top_up/etc.) — left out of scope here since the issue only pointed at lib.rs and test_features.rs, but worth a follow-up doc PR.
• No frontend (lib/contract.ts, hooks) wiring was added — issue scope was the contract entry point itself.

Closes #217