[everton-dgn]

Fixes #29

removeRoute found the entry with toPath(src), which is not always the path that src produced. #29 has the cases and a repro.

• The router records the route path each source file produced, in addRoute and updateRoute. removeRoute looks the path up there and does nothing for a file without a record. Falling back to toPath would bring back the about.ts case, where toPath gives the path of another file.
• removeRoute no longer calls toPath, so BaseFileSystemRouter with only toRoute stops throwing. It also drops the isRoute check, since a recorded file passed it when it was added.
• When addRoute or updateRoute gets a path for a file that differs from the recorded one, it removes the old entry first. The router emits remove for the old path, then add for the new one.

When two files map to the same path (for example about.mdx and about.tsx), removing either one still removes the entry, as before. The other file keeps its record, so a later save of it adds the entry back.

A subclass that calls _addRoute directly bypasses the record, so removeRoute does nothing for its files. Nothing in this repo does that; FlatFileSystemRouter only overrides toPath.

This is independent of #28. Both add a map to BaseFileSystemRouter: #28 records the first source file per path, for ordering, and this one records the path per source file, for removal. They touch the same methods, so whichever lands second needs a small rebase.

The three new tests in test/convention.spec.ts cover:

• a helper module next to a page;
• a custom toRoute through path changes on change and add, an edit that makes it return undefined, and a delete;
• BaseFileSystemRouter with only toRoute.

All three fail on main.