checker: pass EnabledRules to Collect via context

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.

checker/context.go

@@ -0,0 +1,52 @@
+// Copyright 2020-2026 The happyDomain Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package checker
+
+import "context"
+
+type enabledRulesCtxKey struct{}
+
+// WithEnabledRules returns a context carrying the host's per-rule enable map.
+// The SDK server attaches it before calling ObservationProvider.Collect so
+// providers can skip optional work (network calls, paid API hits, …) for
+// rules the host has disabled. A nil map means "run everything".
+func WithEnabledRules(ctx context.Context, enabled map[string]bool) context.Context {
+	if enabled == nil {
+		return ctx
+	}
+	return context.WithValue(ctx, enabledRulesCtxKey{}, enabled)
+}
+
+// EnabledRulesFromContext returns the enabled-rule map attached by
+// WithEnabledRules, or nil if none. RuleEnabled is the usual access pattern.
+func EnabledRulesFromContext(ctx context.Context) map[string]bool {
+	m, _ := ctx.Value(enabledRulesCtxKey{}).(map[string]bool)
+	return m
+}
+
+// RuleEnabled reports whether ruleName is enabled given the host's map.
+// Absent rules default to enabled (nil map or rule not in map), matching
+// the SDK server's evaluate-side semantics.
+func RuleEnabled(ctx context.Context, ruleName string) bool {
+	m := EnabledRulesFromContext(ctx)
+	if m == nil {
+		return true
+	}
+	enabled, ok := m[ruleName]
+	if !ok {
+		return true
+	}
+	return enabled
+}