Fix golangci-lint v2 compliance, make all passes clean

- Fix 314 errcheck violations (blank identifier for unrecoverable errors)
- Fix errorlint violation (errors.Is for io.EOF)
- Remove unused serveL7Route test helper
- Simplify Duration.Seconds() selectors in tests
- Remove unnecessary fmt.Sprintf in test
- Migrate exclusion rules from issues.exclusions to linters.exclusions (v2 schema)
- Add gosec test exclusions (G115, G304, G402, G705)
- Disable fieldalignment govet analyzer (optimization, not correctness)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/proxyproto/proxyproto.go

@@ -50,8 +50,8 @@ const (
 // It reads only the exact bytes of the PROXY header, leaving the connection
 // positioned at the first byte after the header (e.g., TLS ClientHello).
 func Parse(conn net.Conn, deadline time.Time) (Header, error) {
-	conn.SetReadDeadline(deadline)
-	defer conn.SetReadDeadline(time.Time{})
+	_ = conn.SetReadDeadline(deadline)
+	defer func() { _ = conn.SetReadDeadline(time.Time{}) }()
 
 	// Read the first byte to determine version.
 	var first [1]byte