The changes in FStarLang#676 (commit c1e092f) introduced three problems for
non-polymorphic types:

1. The fast-path in visit_node skipped non-polymorphic types that hadn't
   been seen yet, deferring them to their source position. This broke
   DFS dependency ordering: types could appear after the types that use
   them by value.

2. The 'if args <> [] || cgs <> []' guards prevented visit_node from
   recording non-polymorphic types, forcing them to be emitted at their
   source position via visit_decl instead of in DFS order.

3. The visit_decl handler for non-polymorphic DTypes used Gray/Black
   state management and appended the visited declaration, instead of
   delegating to visit_node for proper DFS ordering.

4. Type arguments were visited with the caller's under_ref flag, which
   could trigger full processing of types that are actually used behind
   pointers inside the parameterized type (e.g., slice<T> stores T*).

Fix: restore the old DFS-based ordering by (1) only fast-pathing Black
types, (2) always recording in visit_node, (3) using visit_node from
visit_decl, and (4) visiting type arguments with under_ref=true.

Co-authored-by: Copilot <[email protected]>

The mangle_enum function creates composite names from (type_lid,
enum_lid) to disambiguate enum values across types. When the type at
the use site is a typedef alias rather than the canonical tag type,
mangle_enum produces a different name than at the definition site.

Fix by resolving type abbreviations before calling mangle_enum at
both the EEnum expression and SEnum switch case sites.

Co-authored-by: Copilot <[email protected]>

- DeclOrder.fst: Tests that non-polymorphic types used by value inside
  monomorphised polymorphic types are emitted in correct DFS order.
  Fails without commits d72e7fa and c105d05 (types emitted after
  their users, causing C compilation errors).

- EnumAliasResolve.fst + EnumAliasHelper.fst: Tests that enum values
  are correctly resolved when the type at the use site is a typedef
  alias. Two types share constructor names, causing the DataTypes pass
  to make one an abbreviation of the other. Fails without commit
  8acc4b3 (mangle_enum produces mismatched names for aliases).

Co-authored-by: Copilot <[email protected]>