test(build): close Rust fact and surface mutants

Prove recursive and inline Rust struct discovery, ignored non-Rust/out-of-tree files, source/parse failures, atom and handle collision propagation, and missing form-fact defaults; classify only non-injectable directory-entry errors and syn's named-field invariant.

req: diagnostics/006

req: diagnostics/004

req: surface/008

req: test/021

.cargo/mutants.toml

@@ -20,6 +20,9 @@
 #   non-terminating; sorted, deduplicated, boundary-aware outputs are asserted.
 # - context path words are filtered non-empty before extracting their first char;
 #   `?` and `unwrap` are equivalent under that local iterator invariant.
+# - Rust-fact named fields always carry identifiers by syn's type contract. Per-entry
+#   and recursive read_dir errors cannot be injected portably after the parent opens;
+#   parent-open, source-read, and parse failures remain explicitly proven.
 exclude_re = [
   "test_process_try_wait",
   "test_process_poll_delay",
@@ -41,4 +44,6 @@ exclude_re = [
   "replace \\+= with (?:-=|\\*=) in stylesheet_class_tokens",
   "replace 1 with 0 in stylesheet_class_tokens",
   "replace chars.next\\(\\)\\? with chars.next\\(\\).unwrap\\(\\) in context_type_for_heml_path",
+  "replace entry\\? with entry.unwrap\\(\\) in collect_rust_struct_facts",
+  "replace collect_rust_struct_facts.*\\? with collect_rust_struct_facts.*unwrap\\(\\) in collect_rust_struct_facts",
 ]