Exception handler status_code positional argument and FT response rendering failure

In the tested FastHTML version, exception handlers receive a third positional status_code argument; handlers written to the conventional request, exc signature fail. Returning FT content from this path also mis-renders, while a plain Starlette Response works. Repro context: https://github.com/lgs/iii-fasthtml-frontend/blob/main/docs/ARCHITECTURE.md#11-platform-quirks-discovered-all-worked-around-in-code. Expected: documented/stable handler signature and normal FT rendering on error paths.

Start with the linked docs/ARCHITECTURE.md reproduction and run it against the tested FastHTML version. Trace the exception-handler and error-response paths it exercises; done means the documented handler signature is stable and FT content renders normally on errors, while a plain Starlette Response continues to work.

Written by the indexing model from the issue text.