Add comprehensive usage examples

[roottool]

Description

This PR adds a comprehensive set of TypeScript examples demonstrating safe-formdata usage patterns and best practices.

Boundary Checklist (Required)

safe-formdata enforces a strict trust boundary for FormData.
If any item below does not apply, this change likely does not belong here.

• No interpretation

  • Key names are treated as opaque strings
  • No structural inference ([], ., brackets, paths, etc.)

• No silent behavior

  • No merging, overwriting, auto-fixing, or implicit resolution
  • All boundary violations are reported explicitly

• Boundary respected

  • No validation, coercion, schema, framework conventions, or business logic

📘 Boundary rules & non-goals:
https://github.com/roottool/safe-formdata/blob/main/AGENTS.md

Type of Change

• Bug fix
• Security fix
• Documentation
• Refactoring (no behavior change)
• Tests
• Tooling / CI

Testing

Added Examples:

1. 00-basic.ts - Demonstrates:

   • Basic field access from FormData
   • Validation guard pattern (result.data === null)
   • Explicit type narrowing for string | File values

2. 01-file-upload.ts - Demonstrates:

   • File value handling from multipart/form-data
   • Type narrowing with instanceof File
   • File size and MIME type validation

3. 02-field-presence.ts - Demonstrates:

   • Optional field handling pattern
   • Explicit presence checking with in operator
   • No implicit "optional" field concept

4. 03-error-handling.ts - Demonstrates:

   • Validation failure handling
   • Issue inspection and structured error reporting
   • Duplicate key rejection (boundary enforcement)

5. 04-integration-fetch.ts - Demonstrates:

   • Real-world Request handler integration
   • Parsing FormData from HTTP requests
   • Field validation and HTTP response patterns

CI Updates:

• Added check:type:example script to package.json
• Updated CI workflow to typecheck examples
• Removed unused .tsx file filter

All examples pass TypeScript type checking and demonstrate boundary-compliant patterns.

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f121fda) to head (e68c984).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           22        22           
  Branches         6         6           
=========================================
  Hits            22        22           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.