checker: implement ShareKey to mutualise reverse-DNS lookups across targets

The reverse-DNS observation (zone location, authoritative NS, the PTR RRset,
and the forward-confirm of the effective target) is determined entirely by the
reverse-arpa owner name — i.e. the IP being asked about — never by which
forward domain triggered the check. Implement sdk.ObservationSharer so the host
runs the PTR + FCrDNS lookups once and serves every target that interrogates
the same reverse name, instead of re-querying per record.

The share key derives from the owner name and folds in the declared target and
TTL: these are part of the observation and can change it — when no PTR is
published the effective target (and therefore its forward-confirm) falls back
to the declared value. ShareKey stays a pure function of opts (no network) per
the contract. An empty owner yields "" so the host falls back to per-target
caching.

checker/collect_test.go

@@ -3,6 +3,7 @@ package checker
 import (
 	"encoding/json"
 	"net"
+	"strings"
 	"testing"
 
 	sdk "git.happydns.org/checker-sdk-go/checker"
@@ -158,3 +159,49 @@ func TestResolvePTRInputs_WrongServiceType(t *testing.T) {
 		t.Fatal("expected error for non-PTR service type")
 	}
 }
+
+func TestShareKey_StableAndPrefixed(t *testing.T) {
+	p := &ptrProvider{}
+	opts := sdk.CheckerOptions{"domain_name": "4.3.2.1.in-addr.arpa"}
+	a, err := p.ShareKey(opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, _ := p.ShareKey(opts)
+	if a != b {
+		t.Fatalf("non-deterministic key: %q vs %q", a, b)
+	}
+	if !strings.HasPrefix(a, "ptr:") {
+		t.Fatalf("missing prefix: %q", a)
+	}
+}
+
+func TestShareKey_DiffersByOwner(t *testing.T) {
+	p := &ptrProvider{}
+	a, _ := p.ShareKey(sdk.CheckerOptions{"domain_name": "4.3.2.1.in-addr.arpa"})
+	b, _ := p.ShareKey(sdk.CheckerOptions{"domain_name": "5.3.2.1.in-addr.arpa"})
+	if a == b {
+		t.Fatalf("expected different keys for different owners")
+	}
+}
+
+func TestShareKey_DiffersByDeclaredTarget(t *testing.T) {
+	p := &ptrProvider{}
+	base := sdk.CheckerOptions{"domain_name": "4.3.2.1.in-addr.arpa"}
+	a, _ := p.ShareKey(base)
+	b, _ := p.ShareKey(sdk.CheckerOptions{"domain_name": "4.3.2.1.in-addr.arpa", "expected_target": "host.example.org"})
+	if a == b {
+		t.Fatalf("declared target must affect the key")
+	}
+}
+
+func TestShareKey_EmptyWhenUnresolvable(t *testing.T) {
+	p := &ptrProvider{}
+	sk, err := p.ShareKey(sdk.CheckerOptions{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if sk != "" {
+		t.Fatalf("expected empty key, got %q", sk)
+	}
+}