@rt default all-method registration shadows sibling handlers on same path

Repro: register sibling GET and POST handlers for one path, leaving methods unspecified on one route. @rt registers all methods by default and route ordering causes one sibling to shadow the other. Explicit methods=[GET] / methods=[POST] fixes it. Notes: https://github.com/lgs/iii-fasthtml-frontend/blob/main/docs/ARCHITECTURE.md#11-platform-quirks-discovered-all-worked-around-in-code. Expected: method inference or a conflict error that prevents silent sibling shadowing.

Start at the @rt route-registration entry point and read the linked ARCHITECTURE.md section on platform quirks. Reproduce sibling GET and POST handlers on one path with methods omitted, then verify that method inference or a conflict error prevents silent shadowing while explicit methods still work.

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