Add kernel booleans: union, subtract and intersect for Solid3D

SolidOperation::Boolean runs synchronously, surfaces the kernel's refusal
with a plain reason and leaves the operands untouched, consumes them on
success unless keep_operands, and records one undo step. The lifecycle test
uses planar boxes with exact volumes and covers chaining, refusals, both
formats and undo; an ignored test shows a curved refusal reaching the script.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/cadkernel-body-path.md

@@ -196,3 +196,5 @@ Test it with **planar** operands (millisecond cost, safe in debug) plus one
 30 s script budget and that chained cuts on a drilled body are often refused.
 Do not move booleans to a background thread yet: it would need cancellation and
 undo coordination for a cost that is one to two seconds.
+
+**Status (2026-09-20): implemented.** `SolidOperation::Boolean` and `doc.solids.union`, `subtract` and `intersect` follow the recommendation above, with planar-operand lifecycle testing and an ignored curved refusal test. Every operation this note proposed is now built.