walk only handled vector?/map? and fell through :else for everything else, so
postwalk over a quoted list (a plist) never touched its elements —
postwalk-replace with symbol keys silently no-op'd, which broke
clojure.template/apply-template (found during reitit work). Add list? (rebuild as
a list) and seq? (map over it) branches after the vector/map ones so concrete
collections stay authoritative. Adds walk-spec covering list/seq walking plus a
vector/keywordize regression guard and the apply-template trigger.
