Migrate EnumComponentReferences to use visitor pattern

[darianmiller]

Once both Delphi.Forms.Visitor (#41) and component reference detection
(#40) ship, migrate EnumComponentReferences in Delphi.Forms.References
to use the visitor for traversal.

Reference detection is a clean visitor fit: VisitProperty checks each
fvIdentifier value against the component name index and collects matches.
The component name index is built in a first pass (or in VisitObject).

Scope

• Implement a TReferenceVisitor that builds a component name index
  in VisitObject and detects references in VisitProperty
• Replace ad-hoc recursion in EnumComponentReferences with
  WalkForm(Form, Visitor)
• Public API unchanged

Acceptance criteria

• All reference detection tests pass with identical output
• No ad-hoc recursion remains in Delphi.Forms.References
• Public API unchanged (backward compatible)