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.

net.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 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -14,8 +14,8 @@ import (
 
 // Addr represents a network end point address.
 //
-// The two methods Network and String conventionally return strings
-// that can be passed as the arguments to Dial, but the exact form
+// The two methods [Addr.Network] and [Addr.String] conventionally return strings
+// that can be passed as the arguments to [Dial], but the exact form
 // and meaning of the strings is up to the implementation.
 type Addr interface {
 	Network() string // name of the network (for example, "tcp", "udp")
@@ -38,6 +38,8 @@ type Conn interface {
 
 	// Close closes the connection.
 	// Any blocked Read or Write operations will be unblocked and return errors.
+	// Close may or may not block until any buffered data is sent;
+	// for TCP connections see [*TCPConn.SetLinger].
 	Close() error
 
 	// LocalAddr returns the local network address, if known.