What
Debug log settings that were separate cached lookups (CLAUDE_CODE_DEBUG_LOG_LEVEL, --debug=, --debug-to-stderr, --debug-file, CLAUDE_CODE_DEBUG_LOGS_DIR) now live on one object. Lines are tagged with their session and grouped before flushing, and can be written through the storage layer when one is installed, falling back to plain rotated log files otherwise. A newly started logger takes over the previous one's queued lines instead of losing them.
Evidence
CLAUDE_CODE_DEBUG_LOGS_DIR
Strings lifted out of the shipped bundle, so the claim above can be checked against them.