apply_rules_inner used to handle the "child was rewritten, so the
parent needs new field IDs" case by cloning the parent node, swapping
in the new fields, pushing the clone onto the arena, and returning the
new Id. Every ancestor on the path from the rewrite up to the root was
duplicated this way, with the originals retained as garbage in the
arena.

Switch to in-place mutation: assign `ast.nodes[id].fields = new_fields`
and return the same Id. Rule firings still produce genuinely new nodes
via BuildCtx (their structure differs from the input), but the
ancestor-rebuild spine no longer copies anything.

This is safe because apply_rules_inner already works entirely by Id:
the field snapshot is cloned out before recursing, no &Node references
are held across mutations of the arena, and captures are scoped to a
single rule firing so the now-stable Ids do not break anything.

Memory effect: a desugaring pass that rewrites R leaves of a tree of
average depth d previously appended R*d ancestor clones to the arena.
Now appends 0.

With Ids stable for the lifetime of an Ast, the Node::id field becomes
truly redundant and is removed (along with the Node::id() accessor).
AstCursor switches from caching `node: &Node` to tracking `node_id:
Id` and looking the node up via the arena on each access; ChildrenIter
now yields Ids directly. A new AstCursor::node_id() method gives
callers access to the cursor position by Id.