Modern Swift concurrency

[RISCfuture]

Summary

Adopts the Approachable Concurrency upcoming features for the GameGenerator
executable and modernizes the remaining legacy-concurrency sites so the package
is free of MainActor.run, GCD-as-concurrency, and dead imports.

• Enables NonisolatedNonsendingByDefault and InferIsolatedConformances on the
  executable target.
• Replaces the MainActor.run write-lock around stream writes with an
  actor-isolated append(_:) on GameFinder, and hoists the JSONEncoder into a
  stored, init-once property so the concurrent task-group children serialize their
  output through the actor instead of bouncing to the main actor.
• Removes the unused import Dispatch from GameFinder.
• Reworks the SIGINT (Ctrl-C) handler from a DispatchSource(queue: .main) +
  escaping closure + Self.exit() into an AsyncStream of interrupts bridged
  into a structured withThrowingTaskGroup, with cancellation observed in
  findGames. A Ctrl-C now cancels the search through structured cancellation,
  lets in-flight games finish streaming, and appends the closing ] before
  exiting cleanly. The DispatchSource signal source and signal(SIGINT, SIG_IGN)
  remain as the underlying primitive (there is no native async signal API); only
  their delivery is bridged to structured concurrency.

The command-line interface, JSON output format, and exit code are unchanged.

Verification

• swift build --build-tests succeeds.
• SwiftLint reports 0 violations.
• This package has no unit-test or E2E targets. The SIGINT shutdown path was
  smoke-tested manually: clean exit code 0, well-formed bracketed JSON, and
  in-flight games flushed before exit.

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