ui: Simplify Spaghetti materialization to expensive-op cut points

Reduce the planner to a single pure rule: the output of every expensive
op (now including sort) becomes a cache table and everything else folds
into the next table or the output query. Drop the selected-node and
canvas fan-out rules along with free-op tracking.

Keep the SQL that actually built each cache entry instead of
overwriting it on cache hits, and show only the query body in
Materialization step cards. Label the output card as Standard SQL, since
it always runs as plain SQL for the DataGrid.

Make the PerfettoSQL Next formatter always emit a single query: write
unions as FROM <a> UNION ALL (<b>) ..., repeat steps used more than once
instead of emitting CREATE OR REPLACE PERFETTO TABLE scripts, and error
clearly when a SQL node input would nest a pipe query inside plain SQL.