[everton-dgn]

Fixes #27

The watcher path changed the manifest order that #14 made deterministic: _addRoute moved an updated route to the end and appended new ones.

• _addRoute replaces an existing route in place.
• The router records the source file that first produced each route path. A new route is inserted before the first route whose recorded source sorts after it, which is the position a full scan gives it. removeRoute drops the record.
• _addRoute takes the source as an optional second argument. Without it, a new route is appended, as before.

This also changes the initial scan when two files map to the same path (for example about.mdx and about.tsx): the entry now keeps the position of the first file instead of moving to the end.

The new tests in test/convention.spec.ts update a route, remove and re-add it, and add files in the middle and at the end, comparing router.routes with a fresh scan after each step; a second test covers two files that map to the same path. Both fail on main.