Introduces an optional RulePrecheck interface so rules can declare
prerequisite checks against the current options (e.g. "missing API
key"). POST /definition mirrors GET /definition and adds a
precheck_failures map keyed by rule name, letting a UI fetch the
definition and precheck results in a single round-trip.
Reporters can now read rule output via ctx.States() instead of
re-deriving severity/hints from the raw payload, keeping the rules
screen and the HTML report aligned on a single source of truth.
Move server.go and interactive.go (and their tests) from the root
checker/ package into checker/server/. Plugin and builtin consumers of
the SDK now import only checker/ and no longer drag net/http,
html/template, or the form-rendering code into their artifacts: on
checker-dane.so this drops the binary by ~1.2 MB and removes 170
html/template symbols along with the net/http contribution that came
from the SDK itself.
Breaking for standalone consumers (main.go):
NewServer(p) -> server.New(p)
CheckerInteractive -> server.Interactive
InteractiveRelatedProviders -> server.Siblings
Providers that only satisfy the interactive interfaces structurally
(method set match, no explicit type reference) need no source change;
only main.go has to switch its import path and the constructor name.
