feat(node): progress names the root under way and the roots waiting

`IndexProgress` said "scanning, this many bytes of that many" and nothing
more. A group's roots are walked one after another, so a second directory
added during a large scan showed as the bar jumping back to 0 %. It now also
carries the root being walked and its position in the roots table, the kind
of walk (scan, rescan, reconcile, watch), file counts, and the roots
waiting for the scan lock in order: queued by the initial scan, by a
retarget, and by a plug; dropped when a root is removed.

`GET /api/index-status` answers for every group at once, including a group
still in its initial scan, so a client can show indexing on any page. It
names roots: loopback only, like `current_dir`.

`index_progress` and the handshake ack gain the same counters, still naming
nothing (decision D3): the root is a position in the roots table the member
already opened from the sealed index, and the queue is a count. The pusher
keeps speaking while a root only waits for the lock.

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

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