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/mapper/tail_test.go

@@ -13,7 +13,6 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
-	"tailscale.com/types/ptr"
 )
 
 func TestTailNode(t *testing.T) {
@@ -98,7 +97,7 @@ func TestTailNode(t *testing.T) {
 				IPv4:      iap("100.64.0.1"),
 				Hostname:  "mini",
 				GivenName: "mini",
-				UserID:    ptr.To(uint(0)),
+				UserID:    new(uint(0)),
 				User: &types.User{
 					Name: "mini",
 				},
@@ -140,8 +139,8 @@ func TestTailNode(t *testing.T) {
 				Addresses: []netip.Prefix{netip.MustParsePrefix("100.64.0.1/32")},
 				AllowedIPs: []netip.Prefix{
 					tsaddr.AllIPv4(),
-					netip.MustParsePrefix("192.168.0.0/24"),
 					netip.MustParsePrefix("100.64.0.1/32"),
+					netip.MustParsePrefix("192.168.0.0/24"),
 					tsaddr.AllIPv6(),
 				},
 				PrimaryRoutes: []netip.Prefix{