[Cleanup] (v2.4.3 or later): Remove pre-sharding storage compat code once the format-4 release settles

[kolacheee]

Reminder issue per the approved plan on #4708 (maintainer decision: delete old-format compat code in a later cleanup PR).

Once the sharding release (#4735 + #4738 + #4754, storage format 4) has been out long enough that supporting direct upgrades from pre-sharding installs is no longer needed, a cleanup PR can remove:

• migrateShardedTables / migrateMonolithToShards and the five-state migration classifier in packages/server/src/db/file-backed-store.ts (incl. the .migrating sentinel handling and buildMigrationIndexFromShards)
• the downgrade-artifact quarantine branch (monolith-beside-shards reconciliation)
• the absent-storage-format.json-means-format-2 fallback in both guard twins (scripts/protect-launcher-data.mjs and packages/server/src/routes/updates.routes.ts) once no supported target predates the file
• the corresponding regression cases in scripts/regressions/message-sharding.regression.ts / launcher-format-guard.regression.mjs

Do not do this before at least one (ideally several) stable releases carry the migration - the compat code is what upgrades users through the boundary. The unshard command should stay (it is the documented downgrade escape hatch, not compat code).

Blocked until the sharding release ships and has settled.

[kolacheee]

saving this for 2.4.3 just to ensure Stable users have a full version of time to decide to keep, delete, or move backups.