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/util/test.go

@@ -10,7 +10,7 @@ import (
 )
 
 var PrefixComparer = cmp.Comparer(func(x, y netip.Prefix) bool {
-	return x == y
+	return x.Compare(y) == 0
 })
 
 var IPComparer = cmp.Comparer(func(x, y netip.Addr) bool {