fix(node): a scan keeps its progress while a burst or a plug runs beside it

A watchdog burst wrote the indexer's single `progress` directly. A file
dropped into a folder during a large scan added its size to the scan's
total, then cleared `scanning` when its own hash finished, so the progress
went blank with hours of hashing left. Bursts now keep their own counters,
shown only while no whole-root walk runs.

A plug rescan took no scan lock and walked its root beside an added root's
scan, both resetting the same counters and reading the drive in turn. It
now waits for the lock, and skips the rescan if the root was ejected or
removed while it waited, since the rescan drops the entries before it walks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T6jPTeocXA1BePekdsgPya

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