refactor: extract shared Debouncer/KeyedDebouncer and fold diff render state into an enum master
- Add Debouncer and KeyedDebouncer (Support/) centralizing the cancel-previous + sleep + post-sleep cancellation-check pattern that was hand-rolled in six places across three stores. - Migrate DiffWindowState.selectFile, WorktreeInfoWatcherManager (branch changed, restart, repository worktrees, remote config, line-changes refresh), and PullRequestRefreshCoordinator flush debounce to the helper. - Fix a latent bug in scheduleBranchChanged/scheduleRestart: 'try? await sleep' swallowed the cancellation error and the body ran anyway, so a cancelled debounce fired immediately instead of never. - Remove the dead filesDebounceTasks dictionary (declared and cancelled but never populated). - Replace DiffWindowState.isRenderingDiff + renderError with a single RenderState enum (idle/rendering/failed) so invalid combinations are unrepresentable; view overlay now switches on it with one animation modifier.