fix: apply Live rendering changes during active effects

[Jah-yee]

Problem

isLivePicture was only consulted when presentPicture() ran. Once an overlay was visible, changing Live rendering did not select a new picture source:

• enabling it left the current held frame in place until the next lid cycle
• disabling it left the live stream capturing and updating the overlay

Change

• remember which picture source was selected for the current effect
• reconcile that source with the preference during active polling
• when it changes, stop the old display link and stream, discard the old live texture, and present the effect again with the selected source
• explicitly start the stream when switching from a still picture to live rendering
• invalidate older screenshot continuations so a rapid toggle cannot clear or present over the new source
• generation-gate asynchronous stream setup so a stopped start cannot publish into a newer start

The existing screenshot and live-seeding paths are reused, so behavior outside a mid-effect setting change is unchanged.

Lifecycle edges

• repeated live -> still -> live changes converge on the latest setting
• stopping or sleeping invalidates pending presentation work
• an older stream setup cannot clear a newer startTask, install its receiver, or invalidate the newer filter
• failed setup remains retryable on the next normal start

The capture-cancellation behavior now on main is retained: disabling, sleeping, or stopping the app cancels the underlying screenshot. A normal animated effect end still only discards its cache, so this PR limits its additional guarantee to presentation state rather than changing ScreenSnapshotter ownership.

Validation

• debug build with Swift warnings treated as errors
• release build with Swift warnings treated as errors
• git diff --check
• exercised the generation guards against rapid source changes and stop/start ordering in code review
• retained the cancellation guards from PR Makes the lid effect shut down more cleanly #3 while resolving the rebase onto current main

The change has not been exercised during a physical lid-close cycle.

Coordination

PR #16 also changes stream filter rebuilding, so the two PRs have a small textual conflict in ScreenStreamer. The tested resolution keeps this PR's cancellation and start-generation guards together with #16's fail-closed exclusion helper and post-rebuild filter readiness check.

[Jah-yee]

Closing per Gate-2 (max 2 concurrent PRs per repo). #15 is mergeable_state=dirty with conflicts against #16. Keeping #16 and #17 as the primary clean PRs.