Audit Badger storage dependency upgrade path

[liujuanjuan1984]

Background

The current dependency graph still uses Badger v3 through a forked replacement:

replace github.com/dgraph-io/badger/v3 => github.com/chux0519/badger/v3 v3.2103.3

The declared module is github.com/dgraph-io/badger/v3 v3.2103.2, while upstream v3 has newer patch releases and the current Badger main line is v4.

Why this needs a separate evaluation

Badger is a storage engine dependency, so upgrading it is not equivalent to a normal library patch bump. Major Badger upgrades can affect on-disk data compatibility, migration behavior, value log handling, and operational recovery paths.

This should not be mixed into the current dependency/test-build PR.

Scope to evaluate

• Compare the local fork github.com/chux0519/badger/v3 v3.2103.3 against upstream github.com/dgraph-io/badger/v3.
• Determine why the fork replacement exists and whether the fork still carries required local fixes.
• Evaluate the lowest-risk patch upgrade within v3, including whether upstream v3.2103.5 is compatible with existing persisted data.
• Evaluate whether Badger v4 is viable for this project, including required code changes and data migration/backward-compatibility strategy.
• Add or identify storage regression coverage that exercises existing DB open/read/write/delete/batch paths before attempting a version change.

Current decision

Do not change Badger in the current PR. Track it here as a dedicated dependency/storage migration assessment.