docs: document the cancellation settlement rules

[praizeD10]

Cancellation is the part of the stream lifecycle most likely to catch
integrators off guard — specifically what each party receives, that the
split happens atomically, and that vesting stops permanently from that
point. The code comments in contract.rs describe this well, but there
was no prose a user reading the README would find.

Added a "Worked example: cancel" section alongside the existing
withdraw_amount worked example. It uses the same 1000-unit reference
stream the rest of the docs are built around, walks through both the
clean cancel and the cancel-after-partial-withdrawal case with exact
numbers, and calls out the two rejection conditions
(StreamAlreadyCompleted, AlreadyCancelled) so callers know the
boundaries.

No source code was changed. CI is unaffected.

Closes #148

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