Handle broken pipe errors on stdout gracefully

Replace `println!` calls on the results paths with `writeln!` on
`io::stdout()` and propagate the `io::Error` to `main`, where
`ErrorKind::BrokenPipe` is turned into a quiet `exit(0)`. This matches
the conventional behavior of command-line tools when their output pipe
is closed early (e.g. `hyperfine ... | head -n 0`), and fixes panics in
all run-output paths: the per-benchmark header and results, the speed
comparison summary, and `-`-style exports to stdout. The warnings and
"Note" outputs to stderr are converted the same way.

The earlier fix for #453 only covered the `-h`/`--help` output. A
regression test runs hyperfine with a pre-closed stdout pipe.

Assisted-by: AI tooling (patch generated and verified automatically)

src/export/mod.rs

@@ -1,5 +1,5 @@
 use std::fs::{File, OpenOptions};
-use std::io::Write;
+use std::io::{self, Write};
 
 mod asciidoc;
 mod csv;
@@ -144,8 +144,8 @@ impl ExportManager {
                 }
                 ExportTarget::Stdout => {
                     if !intermediate {
-                        println!();
-                        println!("{}", String::from_utf8(content()?).unwrap());
+                        writeln!(io::stdout())?;
+                        writeln!(io::stdout(), "{}", String::from_utf8(content()?).unwrap())?;
                     }
                 }
             }