Improve cached startup state and keep directory scans responsive

[moreaki]

Summary

On top of fix/combined-directory-monitoring, this branch turns the app into a much better "show something immediately, finish work in the background" experience, and adds guardrails for the slow-path cases.

The main user-facing changes are:

• Latest restores the last known app list on launch from cache in AppDataStore, instead of starting with an empty main window every time.
• Cached update availability is restored too, via Update, App, and UpdateButton, but stale cached updates are treated as non-actionable until a fresh check confirms them.
• Snapshot generation for the main table was moved off the main thread and coalesced in UpdateTableViewController, UpdateTableView+Search, and AppListSnapshot, so large refreshes no longer block the UI while snapshots are rebuilt.
• The Settings locations list no longer repeatedly rescans the same directory count on every redraw; AppDirectoryCellView now uses a shared cached/deduplicated count provider.
• Adding a very broad scan root like /, the home folder, or a whole volume now triggers a warning in AppLocationViewController, so users are nudged toward choosing a narrower app folder instead of accidentally selecting an entire disk.

Under the hood, scan performance and scan behavior were tightened:

• BundleCollector now enumerates candidate .app bundles first and processes them concurrently for larger result sets.
• Source detection no longer needlessly reopens bundles, via BundleCollector, MacAppStoreCheckerOperation, SparkleCheckerOperation, and UpdateCheckCoordinator.
• AppDirectory now performs collection work on a utility queue instead of doing that work inline on a generic serial queue with a misleading name.

The branch also includes diagnostics and failure UX that fit naturally on top of the combined monitoring base branch:

• Observation failures are buffered until a delegate exists in UpdateCheckCoordinator, so startup failures are not dropped.
• Opt-in diagnostics tracing lives in Observable, disabled by default, so we can later build richer app-aware tracing on top of it.
• Real observation/access failures are shown through the polished custom sheet in MainWindowController, and the same component is reused from Settings when a location count hits the same failure mode.
• Strings for the broad-folder warning and permission/access guidance were added in Localizable.strings.

So the PR story is basically:

• base branch fix/combined-directory-monitoring fixes correctness: no startup crash, recursive monitoring, visible access-failure handling
• this branch improves responsiveness and usability on top of that: cached startup state, nonblocking UI refresh, faster/more efficient scanning, smarter Settings behavior, and retained diagnostics support

Verification

• xcodebuild -project Latest.xcodeproj -scheme Latest -configuration Debug CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY= build

I did not run the full test suite end-to-end because the existing test-target dependency issue with CommerceKit / StoreFoundation still applies.