[dajiaohuang]

Related Issue
Fixes #924

Proposed Changes
When same-path, same-host sibling routes have overlapping methods, Starlette's first-match routing can silently shadow a later handler (for example, a default GET/POST route followed by an explicit POST route). FastHTML.add_route now raises ValueError and lists the overlap instead. Same-name/method replacement remains supported, as do disjoint methods and routes on different hosts.

The nbdev source notebook and generated module are synchronized; the API docstring describes the new conflict behavior.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I am aware that this is an nbdev project, and I have edited, cleaned, and synced the source notebooks instead of editing .py or .md files directly.

Additional Information

• python -m pytest --noconftest -p no:cacheprovider tests/test_routing.py: 3 passed on Python 3.11.15; synthetic route registrations only.
• nbdev-export --path nbs/api/00_core.ipynb and nbdev-clean --fname nbs/api/00_core.ipynb completed. git diff --check passed.
• The full notebook/project suite was not run; no unrelated notebooks were executed.