Upgrade net package from Go 1.21.4 to Go 1.26.2

Backport upstream Go standard library changes to TinyGo's net package.

Unmodified files (18) replaced directly from Go 1.26.2 source.
Modified files (22) merged via 3-way diff, preserving all TinyGo
netdev adaptations, TINYGO markers, and embedded-device constraints.

Notable upstream changes included:
- net/http: cookie handling improvements, fs enhancements,
  reverse proxy updates, chunked encoding fixes
- net/http: ServeMux routing and pattern matching updates
- net: IP parsing and MAC address handling improvements
- Various doc link syntax modernization (e.g. [Dial], [Buffers])

TinyGo-only files (netdev.go, tlssock.go) unchanged.
All TINYGO comment markers preserved.

http/transport.go

@@ -1,4 +1,4 @@
-// TINYGO: The following is copied and modified from Go 1.21.4 official implementation.
+// TINYGO: The following is copied and modified from Go 1.26.2 official implementation.
 
 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -13,12 +13,13 @@ package http
 
 import (
 	"io"
+	"sync/atomic"
 )
 
 type readTrackingBody struct {
 	io.ReadCloser
-	didRead  bool
-	didClose bool
+	didRead  bool // not atomic.Bool because only one goroutine (the user's) should be accessing
+	didClose atomic.Bool
 }
 
 type Transport struct{}