Watcher updates move routes out of scan order #27

#14 made the initial scan sort route files, but the watcher path still changes the order. _addRoute removes an existing entry and pushes the new one to the end, and new files are always appended. The Vite plugin calls updateRoute on every change event (src/vite/fs-watcher.ts) and, with types enabled, regenerates the declaration, which is rewritten because its order changed. Saving a route file during development reorders the generated .d.ts and the virtual manifest even when nothing about the routes changed. In a project that commits the declaration, editing a route file that isn't already last in the manifest shows up as a diff, and a later build puts the order back.

Reproduction

Verified with filesystem-routing@0.4.0 from npm on Node 24.22.1. Save as repro.mjs in a project that depends on filesystem-routing@0.4.0 and run node repro.mjs:

import { mkdtempSync, writeFileSync } from 'node:fs'
import { tmpdir } from 'node:os'
import { join } from 'node:path'
import { PageFileSystemRouter } from 'filesystem-routing'

const dir = mkdtempSync(join(tmpdir(), 'routes-'))
for (const name of ['about', 'contact', 'zebra']) {
  writeFileSync(join(dir, `${name}.tsx`), 'export default () => null;')
}
const router = new PageFileSystemRouter({ dir, extensions: ['tsx'] })
const paths = () => router.routes.map(route => route.path).join(' ')

await router.getRoutes()
console.log('scan:          ', paths())

// What the Vite watcher does when about.tsx is saved.
await router.updateRoute(join(dir, 'about.tsx'))
console.log('after update:  ', paths())

writeFileSync(join(dir, 'blog.tsx'), 'export default () => null;')
await router.addRoute(join(dir, 'blog.tsx'))
console.log('after add:     ', paths())

const fresh = new PageFileSystemRouter({ dir, extensions: ['tsx'] })
console.log('fresh scan:    ', (await fresh.getRoutes()).map(route => route.path).join(' '))

Output:

scan:           /about /contact /zebra
after update:   /contact /zebra /about
after add:      /contact /zebra /about /blog
fresh scan:     /about /blog /contact /zebra

Expected

After any sequence of watcher events, router.routes has the same order as a fresh scan of the same files: an updated route keeps its position, and a new route is inserted where the sorted scan would put it.