fix: update drifted file:line citations in Go patterns

Upstream golang/go has shifted several line numbers since citations
were recorded. Updated 6 citations across 3 files:

- documentation.md: server.go:55-57 → 59-62 (ErrWriteAfterFlush)
- documentation.md: transport.go:79-80 → 72-73 (Transports should be reused)
- structs.md: client.go:31-35 → 30-34 (A Client is an HTTP client)
- structs.md: client.go:59-60 → 57-58 (type Client struct)
- style.md: stream.go:280 → 292 (var _ Marshaler)
- style.md: stream.go:280-281 → 292-293 (var _ Marshaler/Unmarshaler)

Verified against golang/go HEAD (depth=1 clone).

patterns/structs.md

@@ -88,7 +88,7 @@ requiring all fields be set before the type is functional.
 **Code examples from source:**
 
 ```go
-// net/http/client.go:31-35
+// net/http/client.go:30-34
 // A Client is an HTTP client. Its zero value ([DefaultClient]) is a
 // usable client that uses [DefaultTransport].
 type Client struct {
@@ -474,7 +474,7 @@ type Signer interface {
 // Transports should be reused instead of created as needed.
 // Transports are safe for concurrent use by multiple goroutines.
 
-// net/http/client.go:59-60
+// net/http/client.go:57-58
 type Client struct {
     Transport RoundTripper  // If nil, DefaultTransport is used.
     // ...