Keep the node index honest across an in-place subtree edit

[ww-mw]

Why

A host that owns its own undo stack — the VS Code custom editor for a
compressed-binary .sldd — does not mutate the tree through deleteNodesById or
the other session forms. It edits nodes in place and then re-registers the source
so nodeIndex describes the result. On a real customer dictionary (75 MB of
data/chunk0.xml, 31k entries) that re-registration is a 3.2 s parse, and the
edit path was paying it twice per committed cell: 7.5 s of host time to express a
one-cell change.

Simply skipping the re-registration is not an option, because it leaves the index
wrong. A node's id is a path, so renaming an entry rekeys the entry and
every descendant; adding or removing a child adds or strips an id the session
hands out. The edit itself looks fine — it is the next edit on an affected row
that fails to resolve, or routes into a detached node.

What

mutateSubtree(root, mutate) performs the same repair at subtree scope:

• snapshots root.flatten() ids before the mutation and rebuilds them after,
  so a rename rekeys descendants too;
• releases the selection for nodes the mutation removed — before minus after
  by identity, so a rename (same nodes, new ids) releases nothing;
• takes the snapshot inside the wrapper on purpose: id is a live getter, so a
  caller snapshotting after the fact would delete exactly the entries it meant to
  keep;
• repairs in a finally, so a mutation that throws still leaves the index
  describing the tree as it stands.

Scope is root.flatten() on both sides, matching what indexSource /
deindexSource index for a whole source. No change-notification is published —
the caller that owns the mutation owns telling its own view.

Tests

test/mutateSubtree.test.ts, 10 cases: a rename rekeys the node and its
descendants while the rest of the document stays indexed and the selection
survives; an added child becomes findable; a removed name-keyed child becomes
null; a removed positional child stops being handed out even though a sibling
inherits its id; a selection on a removed node is released; the wrapper returns
the mutation's value; the index is repaired even when the mutation throws.

npm run verify green: 4093 tests, dist rebuilt, check:pack and check:leak OK.