docs: add feature comparison vs competitor form libraries

[phmatray]

Summary

• Add COMPARISON.md with a detailed feature comparison matrix covering FormCraft vs Blazor EditForm, Blazored.FluentValidation, and MudBlazor Forms
• Include side-by-side code examples showing boilerplate reduction with FormCraft's fluent API
• Add "When to use each solution" guidance for fair, objective positioning
• Add a "How FormCraft Compares" summary section to README.md linking to the full comparison

Changes

• COMPARISON.md (new) — Full competitive analysis document with:
  • Feature comparison matrix table across 6 categories (form definition, type safety, validation, dynamic forms, layout, security)
  • Code comparison for a registration form: EditForm (~45 lines), MudBlazor (~35 lines), FormCraft (~15 lines)
  • Key differentiators section (type-safe fluent API, automatic rendering, field dependencies, MudBlazor integration, security)
  • "When to use each solution" decision table
  • Notes on combining solutions (FormCraft does not require all-or-nothing adoption)
• README.md — Added a compact comparison summary table before the Installation section

Testing

• Verify all markdown renders correctly on GitHub
• Confirm COMPARISON.md link in README resolves properly
• Review comparison claims against current FormCraft feature set

Fixes #81