Fix the following issue:

The route serializer is used by links, redirects, and active-navigation checks, but its current implementation mutates the objects callers pass in while choosing a route. That makes reusable route state and view-model data lose their `route` selector after a URL is generated, and it makes it unsafe to reuse the same options object for a later operation. Tighten the public route serialization contract across `can.route.param`, `can.route.url`, and `can.route.current` without changing the established hashbang format.

`can.route.param(data)` must choose the best registered route, preserve explicit values including `0`, `false`, and empty strings, append unmatched properties using the existing query encoding, and leave the complete input object unchanged, including its `route` selector and nested values. `can.route.url(data, merge)` must return the configured root plus the parameterized fragment; with `merge=true`, combine the current URL state with the supplied overrides so omitted current properties survive and supplied falsey values win, while never mutating the override object. With `merge=false` or omitted, use only the supplied data. `can.route.current(data)` must compare the supplied constraints with the canonical current route state, distinguish a missing constraint from an explicitly falsey one, and leave the comparison object unchanged. Existing route selection, encoding, and deparameterization behavior must remain compatible.