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.
Providers can now skip optional work (network calls, paid API hits) for
rules the host has disabled. The /collect request grows an EnabledRules
field, and server.handleCollect attaches it to the context with
WithEnabledRules; providers read it via EnabledRulesFromContext or the
per-rule RuleEnabled helper.
Drains in-flight requests within a 10s timeout and stops the load-average
sampler before returning. Callers needing custom signal handling can still
opt out via Handler() and run their own http.Server.
Registered on the default FlagSet; ListenAndServe intercepts it and
exits 0/1 after probing /health on the -listen address. Lets checker-*
Dockerfiles add HEALTHCHECK without any main.go change, even though
the runtime image is scratch (no shell, no curl, no wget).
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.
