test(build): close residual helper mutants

Prove compact Rust type and vector facts plus default template paths, alias collision suppression, data parameter prefixes, keyed descendants, and absent Cargo roots. Classify only hemplate's infallible surface parser seam.

req: diagnostics/006

req: diagnostics/004

req: codegen/005

req: test/021

.cargo/mutants.toml

@@ -10,6 +10,7 @@
 #   are behaviorally equivalent at these validated boundaries.
 # - hemx-build source inspection delegates to hemplate's currently infallible
 #   Surface parser; file I/O and invalid Rust-context errors remain explicitly proven.
+#   The direct surface_for_heml_source unwrap mutant is equivalent for the same seam.
 # - AppBuilder reuses that same parser seam. Directory-open and recursive errors are
 #   proven, while a per-entry readdir fault cannot be injected portably after a
 #   successful read_dir; its unwrap mutant is classified as infrastructure-only.
@@ -35,6 +36,7 @@ exclude_re = [
   "replace String::from_utf8.* with String::from_utf8.*unwrap\\(\\) in InteractionForm::parse_multipart",
   "replace HeaderValue::from_str.*runtime_js_hash.* with HeaderValue::from_str.*unwrap\\(\\) in <impl IntoResponse for RuntimeJs>::into_response",
   'replace "runtime hash is a valid ETag" with "" in <impl IntoResponse for RuntimeJs>::into_response',
+  "replace build_ast.* with build_ast.*unwrap\\(\\) in surface_for_heml_source",
   "replace surface_for_heml_source.* with surface_for_heml_source.*unwrap\\(\\) in diagnostics_for_heml_source",
   "replace surface_for_heml_source.* with surface_for_heml_source.*unwrap\\(\\) in generated_targets_for_heml_source",
   "replace surface_for_heml_source.* with surface_for_heml_source.*unwrap\\(\\) in template_context_facts_for_heml_source",