Eighth write tool in the catalog. Required args: node_id, kind,
name, x, y, width, height. Optional: fill_hex. Builds a fresh
node with a non-colliding id and swaps it into the same parent
slot the target node currently occupies, preserving sibling
order.
Bounded scope: only leaf-style fields land on the replacement.
Full subtree replacement requires a JSON Node parser that
doesn't live on this side yet. The current contract matches TS
`replace_node` for primitives, minus children.
Apply path follows the same pre-validate-then-mutate discipline
as `update_node` and `move_node`: kind / geometry / fill_hex /
target existence / id space — every check before any mutation.
A bad fill_hex never leaves the document half-touched (covered
by `apply_mcp_command_replace_node_atomic_on_invalid_fill_hex`).
Tests live in `mcp/replace_node_tests.rs` (matches the
`copy_node_tests.rs` sibling pattern; keeps `write_tools_tests.rs`
under cap).
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