fix: partition DNS cache by EDNS Client Subnet

With cache_enable = true, one cache entry was shared by every client
asking the same name against the same upstream: the cache key
({Qtype, Qclass, Name, Upstream}) and the osResolver hot-cache/singleflight
key ("name:qtype:") both ignored the EDNS Client Subnet (ECS). A response
tailored for subnet A was therefore served to subnet B.

A cached answer's records are scoped to the network that generated them
(RFC 7871 §7.3), so sharing them across subnets returns the wrong
CDN/policy answer. Rewriting only the ECS option on the shared answer is
worse: forwarders that validate the echoed ECS (e.g. dnsmasq with
add-subnet) then accept the wrong-subnet answer instead of rejecting it as
a mismatch.

Partition both cache paths by a canonical ECS tuple (family, source-prefix,
masked address) via the new dnscache.CanonicalECS: the LRU key gains an ECS
field and the singleflight/hot-cache key appends the canonical ECS. Same
subnet still shares an entry; different subnets (or address families) never
do. Only a request with no ECS option collapses to the shared empty
partition; a carried /0 keeps its own family-scoped token, since it is
forwarded with an ECS option and must stay distinguishable from a no-ECS
query (RFC 7871 §7.3.1). SetCacheReply no longer touches ECS and only
reconciles the EDNS Cookie.

Adds real cache-path regression tests (LRU and osResolver hot cache) that
serve a different A record per subnet and verify the second subnet never
receives the first's record.

Fixes https://github.com/Control-D-Inc/ctrld/issues/324

resolver.go

@@ -19,6 +19,8 @@ import (
 	"golang.org/x/sync/singleflight"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsaddr"
+
+	"github.com/Control-D-Inc/ctrld/internal/dnscache"
 )
 
 const (
@@ -378,8 +380,10 @@ func (o *osResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error
 	domain := strings.TrimSuffix(msg.Question[0].Name, ".")
 	qtype := msg.Question[0].Qtype
 
-	// Unique key for the singleflight group.
-	key := fmt.Sprintf("%s:%d:", domain, qtype)
+	// Unique key for the singleflight group. The EDNS Client Subnet is part of
+	// the key so subnet-specific answers are neither coalesced nor hot-cached
+	// across different subnets (RFC 7871 §7.3).
+	key := fmt.Sprintf("%s:%d:%s", domain, qtype, dnscache.CanonicalECS(msg))
 
 	// Checking the cache first.
 	if val, ok := o.cache.Load(key); ok {