Improve immediate closure scope inheritance

Follow-up to #44. The current implementation only supports MainActor.assumeIsolated with a hardcoded check. This issue tracks making the feature more general and user-extensible.

Tasks

• Add more known immediate closure functions — withCheckedContinuation, withTaskGroup, withUnsafeBufferPointer, etc. Audit stdlib/concurrency APIs for functions that execute closures synchronously before returning.

• Better matching — current matcher checks base.description == "MainActor" which is a string comparison on the syntax tree. Consider matching on the full qualified path and supporting chained member access.

• User-side configuration or attribute-based system — allow users to declare their own immediate closure functions without modifying the library. Options:

  • Package-level config file listing immediate functions
  • A Swift attribute/annotation on function declarations (e.g. @ImmediateClosure on specific parameters)

• Per-parameter granularity — move from "this function is immediate" to "this parameter is immediate". Functions can have multiple closure parameters where some execute immediately and some are escaping (e.g. UIView.animate(withDuration:animations:completion:)). Requires #43 first to model multiple trailing closures in the syntax tree.

Start by reading the current MainActor.assumeIsolated handling and its syntax-tree matcher, then review #43 before designing per-parameter support. Audit the listed standard-library and concurrency APIs, and compare configuration and attribute-based options. Done means immediate closures are matched beyond the hardcoded case, user-extensible, and granular enough for functions with multiple closure parameters.