refactor: Build dense MAST forests through finalization

[huitseeker]

Closes #3325. On top of #3311. Re-do of #3326 after misclick

Dense MastForest values now leave normal construction in canonical node order. Builders may still append while they are building, but they must call finalization before returning a MastForest.

This PR adds DenseMastForestBuilder for dense construction. finish() and finish_with_id_map() call MastForest::from_parts_with_id_map(), which orders MastForest.nodes as external nodes, then basic blocks, then internal nodes. External nodes are sorted by digest. Children stay before parents. The returned ID map uses the final node IDs.

MastForestMerger now builds through that builder and remaps root maps after finalization. The assembly MastForestBuilder finalizer now applies the final ID map before it builds source and debug metadata.

The old append API is no longer available in normal builds. add_to_forest() and make_root() are limited to tests and arbitrary.

Serialization now expects the in-memory order to be valid. It no longer rebuilds a canonical forest before writing. The dependency commitment reads the external node prefix directly.

Public examples now use DenseMastForestBuilder, so docs do not point callers back to the append API.

[huitseeker]

Closing this in favor of #3334.