fix(gpx): output valid <wpt> tag for home waypoint (resolves #20) (#31)

* feat: Add GPS home waypoint to GPX export

- Remove underscore prefix from home_coordinates parameter (now used)
- Add home position waypoint to GPX files using first available H frame
- Waypoint includes name, symbol (Flag), and description for mapping tools
- Home coordinates are rendered before track segment for proper GPX structure
- Provides visual reference point in GPS mapping applications
- Backward compatible: waypoints are optional in GPX 1.1 spec

Resolves #20

* fix(gpx): output valid <wpt> tag for home waypoint

- Add missing closing bracket to <wpt> tag in GPX export
- Home waypoint now produces valid XML, fixing Google Maps and GPX tool import errors
- Resolves #20

* test: add comprehensive unit tests for GPX export home waypoint (TempDir auto-cleanup, 8 cases, 100% coverage)

Author: nerdCopter