fix: insert sketches and datums at the history cursor

[jeffglousher]

Summary

• New sketches and construction planes now insert at the build cursor, same as solids after fix: harden threads, sketch constraints, and modeling history #47.
• Rolling back and starting a sketch no longer appends to the end or re-activates later features.

Test plan

• new_sketch_is_inserted_at_the_build_cursor covers rollback → sketch while a later sketch exists
• cargo test -p nbcad-sketch --test solid_history

[jackControls]

Test does not compile — please fix before merge.

Local run of cargo test -p nbcad-sketch --test solid_history fails:

error[E0507]: cannot move out of `feature.name` which is behind a shared reference
   --> crates/sketch/tests/solid_history.rs:348
348 |         .map(|feature| feature.name)

FeatureTree::iter() yields &Feature and Feature.name: String (crates/core/src/feature.rs), so the test file never compiles. Existing tests in the same file use feature.name.as_str() (line 300) or feature.name == name (line 176). Fix: feature.name.clone() (or .as_str()).

CI didn't catch this because mcp-server.yml only tests mcp-server / nbcad-export / xtask — it never compiles crates/sketch's integration tests. Please:

1. Fix the line (.clone())
2. Run cargo test -p nbcad-sketch --test solid_history and confirm green
3. Tick the test-plan checkbox and push

The change itself (insert_at_rollback for sketches/planes) reviewed fine.

[jeffglousher]

Fixed — feature.name.clone() so the iterator no longer moves out of &Feature.

cargo test -p nbcad-sketch --test solid_history — 4 passed.