defaults: built-in baseline is dual-stack (IPv6 + IPv4), not IPv6-only

BuiltinFamilyDefaults() now returns {WantV6: true, WantV4: true}. Pods
that want a single family explicitly opt out via the
flock.fritzlab.net/ipv4 (or ipv6) annotation, or the operator narrows
the default at the node level via NodeConfig.Spec.Defaults.

Annotation precedence is unchanged: pod annotation > NodeConfig defaults
> built-in baseline. Tests updated to reflect the new baseline; the
"opt out of v4" path now has explicit coverage.

Docs updated:
  - NodeConfig.Spec.Defaults Go doc + CRD descriptions reflect the new
    baseline and its overrides
  - README opening framing softened from "IPv6-first" to "dual-stack,
    IPv6-friendly"; example pods + spec.defaults table flipped to
    treat dual-stack as the default and v6/v4-only as overrides
  - README NetworkPolicy line in the comparison table flipped to
    "yes (nftables)" since v1 enforcement shipped
  - Limitations note about IPv4-only destinations rewritten — every
    pod has v4 by default now, so the question is whether your IPv4
    pool is routable beyond your network

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

pkg/agent/ipam.go

@@ -69,8 +69,9 @@ type AllocRequest struct {
 	Namespace   string
 	Pod         string
 	// WantV6 / WantV4 are the post-merge address family selection (pod
-	// annotation > NodeConfig.Spec.Defaults > built-in baseline). At least
-	// one MUST be true; Allocate rejects the request otherwise.
+	// annotation > NodeConfig.Spec.Defaults > built-in baseline of
+	// dual-stack). At least one MUST be true; Allocate rejects the request
+	// otherwise.
 	WantV6 bool
 	WantV4 bool
 	// AnnCIDR6 / AnnCIDR4 come from the cidr6 / cidr4 annotations. Empty