"Locate Logic Pro Library.bundle" on every launch + all Sound Library downloads fail (even after full reinstall)

Setup: MacBook Pro M1, 16 GB RAM, macOS 26.6.1 (25G76), Logic Pro 12.3.1 (6682), installed via the App Store. Library bundle is in the default location, ~/Music/Logic Pro Library.bundle (~73 GB).

Symptoms (both 100% reproducible):

1. Every launch, Logic shows "Logic Pro cannot find the Logic Pro Library.bundle. Please locate the bundle, or create a new empty one." This happens even right after successfully locating/re-selecting the correct bundle, or after creating a brand-new empty one.
2. Any Sound Library download (Essential Sounds, individual packs) completes the network transfer fine, then fails immediately with a generic "The download has failed" error.

What I've already tried, with no change in either symptom:

• Freed up disk space (was low due to an un-thinned local Time Machine snapshot) — now 100+ GB free
• Disabled Wi-Fi proxy auto-discovery (was flooding logs with unrelated network errors)
• Repeatedly used Locate and "Create New" on the library bundle
• Reset Logic's preferences (com.apple.logic10.plist) to a clean, freshly-generated file
• Updated macOS (25F84 → 25G76)
• Removed two long-expired Apple certificates from the System keychain (WWDR CA, Software Signing) — unrelated as it turned out, but ruled out
• Full clean uninstall and reinstall of Logic Pro itself — deleted the app, all caches, all preferences, the shared content database (~/Music/Audio Music Apps/Databases/), and the library's Package Definitions manifest, then reinstalled fresh from the App Store. Sound content itself (samples/patches/loops/projects) was never touched. Bug reproduced identically on the fresh install.

Comparison point: GarageBand, same Mac, same Apple ID, same network, downloads content successfully every time.

Diagnostic evidence: macOS itself has flagged Logic three separate times for excessive resource use during these failed downloads/launches (Console → Diagnostic Reports). All three point into the same code path, just tripping different limits:

• Two "disk writes" violations (~2.1 GB written in a runaway loop) with the heaviest stack in MAUniversalContent/MASearchKit → CoreData → SQLite (btreeOverwriteContent → pager_write)
• One fresh "cpu usage" violation (90 of 98 seconds of CPU) with the heaviest stack in Alchemy's preset scanner: Alchemy2Shell::presetScanner → AlchemyZenDB::scanForNewPresets → purgeUnusedDynamicAttributes → updateFTSAttributesForAllPresets → SQLite FTS3 index rebuild

So this looks like Logic's content-cataloging/search-index engine (Alchemy's preset DB / the shared content database) getting stuck in an endless rebuild loop, either burning disk I/O or CPU depending on where it's caught — and it happens on a completely clean install with fresh prefs, fresh caches, and a fresh content database, which rules out leftover corrupted app state as the cause.

Question for the forum: has anyone hit this specific "stuck re-indexing/scanning" behavior in Alchemy or the shared content database, and found a fix short of starting over with a brand-new (empty) library bundle? Trying to avoid abandoning 73 GB of an existing library if there's a known way to force a clean re-index.