fix(node): the MNP root path never reloaded, and my first repair made it worse

The previous commit was the wrong fix. `add_root` did leave the running node
unchanged, but editing the live `RootSet` in place — which is what I did — is
wrong in the other direction.

`DirectoryIndexer.retarget` decides what to scan by diffing the names it
already holds against the ones it is handed, and `_retarget_indexer` hands it
`groups_ctx[gid]["roots"]`: the very object the op had edited. So the new root
sat on both sides of the comparison, nothing was scanned, and the directory
would have appeared in the table permanently empty. `_reload_config_inner`
diffs the same way and would have concluded nothing changed. `remove_root` had
the same shape and would have kept serving a removed directory's files.

The real defect is that two front doors did different things. `ui/app.py` has
always fired the daemon's `reload_fn` after these ops, which re-reads node.toml
and builds a *fresh* set; the MNP path retargeted a stale object instead. That
asymmetry is exactly what `ops.py` exists to prevent, and it is why the bug
survived until an operator added a directory from a browser — the loopback path
worked all along.

So: the ops leave the live set alone, `_retarget_indexer` asks the daemon to
reload, and `update_root` keeps editing in place because flags change no files
and the synchronous upload handler reads that object on the next request.

The tests now check the files rather than `describe()`, which proves nothing
about whether anything was scanned. One of them demonstrates the failure mode
instead of describing it, so the rule is checkable and will say so if
`retarget` ever changes. Two more cover the seam itself — that the MNP path
reloads, and that a context with no daemon still retargets.

Diagnosed by reading the running node's journal rather than the source: the
first add logged "Reloading config" and a rescan, the two later ones logged
neither. I should have looked there before the first attempt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pvMdvLBG92jyhvD5pD6us

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