REC-124: drop empty impl DomainChunkBuild {}

src/domain.rs:570 was an empty inherent impl block with no methods. The
struct itself (src/domain.rs:563-567) is live: constructed by
build_domain_chunks (src/domain.rs:750-758) and re-exported from lib.rs:31.
Only the vacant impl block, which added nothing, is removed.

Kill attempt: git grep -n "DomainChunkBuild" origin/main -- returned the
struct definition, this empty impl, its one constructor, and the lib.rs
re-export — no method call sites, since there were no methods to call.

No collision check needed: this repo has a single checkout on main with no
other worktrees.

Verified: cargo check -p gamecult-geometry-csg --locked succeeds; Cargo.lock
unchanged.

REC-124

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

src/domain.rs

@@ -567,8 +567,6 @@ pub struct DomainChunkBuild {
     pub manifest: SelectedCutManifest,
 }
 
-impl DomainChunkBuild {}
-
 #[derive(Clone, Debug)]
 pub struct TriangleChunk {
     pub key: DomainKey,