…D route precedence (Kludex#3182)

fix Kludex#3193: staticfiles.is_not_modified() used tag.strip(' W/') which operates
on a character set, incorrectly trimming ETags ending in 'W' or '/'. Replaced
with a normalize_etag() helper that uses str.startswith('W/') for prefix removal.

fix Kludex#3182: Route.__init__ adds HEAD implicitly when GET is defined, causing the
GET route to steal HEAD requests before any explicit HEAD route can match.
Tracked _explicit_methods and return Match.PARTIAL for implicit HEAD so explicit
HEAD routes registered at the same path can take precedence.