feat: add QoraClient.mutate() and context.mutate() for direct cache writes (#20)

[donfreddy]

Closes #20

Description

Adds QoraClient.mutate<T>(key, data) and context.mutate<T>(key, data) for optimistic cache writes from anywhere in the widget tree.

Changes

• QoraClient.mutate<T>(key, data) : convenience alias for setQueryData -- write directly to cache without a fetcher
• context.mutate<T>(key, data) : BuildContext extension shorthand, triggers UI updates for all active subscribers
• Tests (4 test cases): stream update on mutate, alias equivalence, new entry creation, multiple subscribers
• Updated extension docs to showcase context.mutate