Test report and trees stdout write failures (closes #30)

report and trees already checked every stdout write and the final
flush. run now takes the stdout it hands to them, so tests pass a
closed file or a failing writer instead of swapping os.Stdout: a
closed stdout exits 1 with a one-line diagnostic, and the writer's
error reaches the caller.

README "Error handling" now states the two cases that never reach
sfdupes as a failed write: a pipe reader that exits early ends the
process with SIGPIPE, as with cat; and stdout closed with >&- is
replaced by /dev/null by the Go runtime before main runs, so the run
succeeds.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- test stdout write failures in `report` and `trees`; README states that
+  `| head` ends sfdupes by `SIGPIPE` and `>&-` writes to `/dev/null`
+  (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/30)
+
 - escape tabs, newlines, carriage returns and backslashes in report,
   trees and warning paths; the root directory's path is `/`
   (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/7)