Migrate ComputeStatistics to use visitor pattern

[darianmiller]

Once Delphi.Forms.Visitor (#41) ships, migrate ComputeStatistics in
Delphi.Forms.Statistics to use the visitor for traversal instead of its
ad-hoc recursive walker.

Statistics computation is a natural fit for the visitor: VisitObject
increments component counts and tracks depth, VisitProperty counts
properties and accumulates name frequencies, VisitValue tallies value
kind distribution.

Scope

• Implement a TStatisticsVisitor that accumulates all statistics
  fields in VisitObject, VisitProperty, and VisitValue
• Replace ad-hoc recursion in ComputeStatistics with
  WalkForm(Form, Visitor)
• ComputeStatistics public API unchanged
• Both single-form and batch overloads still work

Acceptance criteria

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