all: upgrade to Go 1.26rc2 and modernize codebase

This commit upgrades the codebase from Go 1.25.5 to Go 1.26rc2 and
adopts new language features.

Toolchain updates:
- go.mod: go 1.25.5 → go 1.26rc2
- flake.nix: buildGo125Module → buildGo126Module, go_1_25 → go_1_26
- flake.nix: build golangci-lint from source with Go 1.26
- Dockerfile.integration: golang:1.25-trixie → golang:1.26rc2-trixie
- Dockerfile.tailscale-HEAD: golang:1.25-alpine → golang:1.26rc2-alpine
- Dockerfile.derper: golang:alpine → golang:1.26rc2-alpine
- .goreleaser.yml: go mod tidy -compat=1.25 → -compat=1.26
- cmd/hi/run.go: fallback Go version 1.25 → 1.26rc2
- .pre-commit-config.yaml: simplify golangci-lint hook entry

Code modernization using Go 1.26 features:
- Replace tsaddr.SortPrefixes with slices.SortFunc + netip.Prefix.Compare
- Replace ptr.To(x) with new(x) syntax
- Replace errors.As with errors.AsType[T]

Lint rule updates:
- Add forbidigo rules to prevent regression to old patterns

hscontrol/types/change/change.go

@@ -333,7 +333,7 @@ func NodeOnline(nodeID types.NodeID) Change {
 		PeerPatches: []*tailcfg.PeerChange{
 			{
 				NodeID: nodeID.NodeID(),
-				Online: ptrTo(true),
+				Online: new(true),
 			},
 		},
 	}
@@ -346,7 +346,7 @@ func NodeOffline(nodeID types.NodeID) Change {
 		PeerPatches: []*tailcfg.PeerChange{
 			{
 				NodeID: nodeID.NodeID(),
-				Online: ptrTo(false),
+				Online: new(false),
 			},
 		},
 	}
@@ -367,7 +367,7 @@ func KeyExpiry(nodeID types.NodeID, expiry *time.Time) Change {
 
 // ptrTo returns a pointer to the given value.
 func ptrTo[T any](v T) *T {
-	return &v
+	return new(v)
 }
 
 // High-level change constructors

hscontrol/types/change/change_test.go

@@ -16,8 +16,8 @@ func TestChange_FieldSync(t *testing.T) {
 	typ := reflect.TypeFor[Change]()
 	boolCount := 0
 
-	for i := range typ.NumField() {
-		if typ.Field(i).Type.Kind() == reflect.Bool {
+	for field := range typ.Fields() {
+		if field.Type.Kind() == reflect.Bool {
 			boolCount++
 		}
 	}