fix(build): recognize exact Hemlate derives

Parse derive paths structurally so qualified Hemlate derives provide template context while similarly named derives fail closed. Prove hyphen/underscore template type mapping, empty and non-UTF-8 paths, Cargo-root lookup, Rust source failures, and derived field facts through the public inspection entry point.

req: diagnostics/006

req: surface/008

req: test/021

.cargo/mutants.toml

@@ -18,6 +18,8 @@
 #   is externally equivalent while create/update/no-op behavior is mutation-proven.
 # - stylesheet_class_tokens loop-progress mutants are deterministically
 #   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.
 exclude_re = [
   "test_process_try_wait",
   "test_process_poll_delay",
@@ -38,4 +40,5 @@ exclude_re = [
   "replace match guard error.kind\\(\\) == io::ErrorKind::NotFound with true in write_if_changed",
   "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",
 ]