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/batcher_lockfree.go

@@ -16,7 +16,6 @@ import (
 	"github.com/rs/zerolog"
 	"github.com/rs/zerolog/log"
 	"tailscale.com/tailcfg"
-	"tailscale.com/types/ptr"
 )
 
 // LockFreeBatcher errors.
@@ -151,7 +150,7 @@ func (b *LockFreeBatcher) RemoveNode(id types.NodeID, c chan<- *tailcfg.MapRespo
 	// No active connections - keep the node entry alive for rapid reconnections
 	// The node will get a fresh full map when it reconnects
 	nlog.Debug().Caller().Msg("node disconnected from batcher, keeping entry for rapid reconnection")
-	b.connected.Store(id, ptr.To(time.Now()))
+	b.connected.Store(id, new(time.Now()))
 
 	return false
 }