fix(node): an added root is served before it is scanned

Adding a large directory to a running group made the reload await the
scan of the new root before putting the new RootSet in the group's
context, holding _reload_lock the whole time. For the hours a large drive
takes to hash, the node served the old set:

- a file request under the new root got None from entry_abs_path and the
  handler died on None.exists() without replying;
- a writable/removable toggle answered with the live table, still the
  old one, so the directory vanished from the operator's settings;
- reconcile saw every file the scan had not reached as a missed event and
  hashed it again on the same executor, rewriting progress under the scan.

retarget now applies the set, the roots table and the watcher first, and
with wait=False scans the added roots in the background; the daemon swaps
ctx["roots"] before calling it. A scan lock shared by the initial scan,
added-root scans and reconcile makes the reconcile loop sit out a running
scan without backing off. Every transport site that resolves an entry
answers ROOT_NOT_SERVED instead of crashing, and a delete keeps the entry.

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

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