downloader: add torrents from disk after snapshot sync stage

[anacrolix]

Summary

• Moves AddTorrentsFromDisk call from initDownloader (at startup, before any sync) to an afterSnapshotDownload callback invoked once the snapshot sync stage completes on the first sync cycle.
• This eliminates false-positive incomplete torrent reports that occurred because we were registering on-disk snapshots before the sync stage had a chance to complete them.
• The afterDownload callback is threaded through StageSnapshotsCfg, NewDefaultStages, and NewPipelineStages with a nil default so callers that don't need it are unaffected.
• Incomplete snapshot warnings after this callback are now flagged as unexpected (sync just finished), with a comment noting the preverified-set edge case that remains unresolved.

Background

Issue #19435 first identified this problem: AddTorrentsFromDisk was being called before initial sync status was checked, causing old or stale .torrent files on disk to interfere with the sync stage's ability to fetch the correct (preverified) versions of snapshots.

PR #19316 by @Giulio2002 addressed a related symptom — converting the fatal "snapshot already loaded with different infohash" error in addPreverifiedSnapshotForDownload into a warning — but this was treating a symptom rather than the root cause.

Other related changes to this area:

• PR Final downloader cleanup #18056 — "Final downloader cleanup": Changed AddTorrentsFromDisk to only add completed snapshots (anything incomplete is assumed a mistake), and removed it as a default at startup.
• PR Quick fix for --downloader.verify assertion #18270 — "Quick fix for --downloader.verify assertion": Fixed a panic in addCompleteTorrentFromMetainfo when called from AddTorrentsFromDisk on already-completed files (--downloader.verify asserts for already completed files #18165).
• PR Fix a collection of downloader, snapshot sync and torrent related issues #15043 — "Fix a collection of downloader, snapshot sync and torrent related issues": Earlier broad fix addressing how disk-loaded torrents interact with preverified snapshots.
• PR Require torrents added from disk to complete on downloader.verify #16655 — "Require torrents added from disk to complete on downloader.verify": Enforced completion check for disk-loaded torrents during verify.

@mh0lt — please take a look, particularly at the scenario where extra snapshots land on disk that weren't part of the sync set (e.g. after an upgrade or downgrade that changes the preverified set).

[anacrolix]

Manually dispatched QA - Snapshot Download on this branch to exercise the scenario where snapshots are already present on disk (e.g. after an upgrade/downgrade), which is the primary case the new afterSnapshotDownload callback is meant to handle.