Migrate NormalizeForm to use visitor pattern

[darianmiller]

Once Delphi.Forms.Visitor (#41) ships, migrate NormalizeForm in
Delphi.Forms.Normalize to use the visitor for traversal instead of
its ad-hoc recursive NormalizeObject procedure.

The visitor walks the tree; the normalization logic just checks each
object's properties and removes matching ones. This eliminates the
custom recursion through children and collection items.

Scope

• Implement a TNormalizeVisitor that removes properties per rules
  in VisitObject
• Replace NormalizeObject recursion with WalkForm(Form, Visitor)
• NormalizeForm public API unchanged -- still takes TFormFile and
  TFormNormalizeRules
• All existing normalization tests still pass with identical behavior

Acceptance criteria

• All 16 normalization tests pass unchanged
• No ad-hoc recursion remains in Delphi.Forms.Normalize
• Public API unchanged (backward compatible)