Open files to hash without blocking and reject non-regular ones (closes #100)

A regular file replaced by a FIFO after the walk statted it made the
hash phase's open wait forever for a writer, so the scan never ended
and kept the lock. The hash and content phases now open each file with
O_NONBLOCK and check with fstat that it is still a regular file;
anything else is a per-path error, warned about and skipped. The new
test replaces a walked file with a FIFO before the hash phase reads it.

Model: opus-5-5

TODO.md

@@ -34,10 +34,6 @@
   statted is warned about and skipped instead of hanging the scan (2026-10-14,
   https://git.eeqj.de/sneak/sfdupes/issues/100)
 
-- on a terminal, a warning for the last file of a phase no longer erases the
-  phase's finished bar: the bar is drawn again below the warning (2026-10-14,
-  https://git.eeqj.de/sneak/sfdupes/issues/102)
-
 - `scan` records mtime to the nanosecond, as whole seconds in `mtime` plus
   `mtime_nsec`, and compares it at that resolution, so a same-size rewrite
   within the same second is re-hashed (2026-10-07,

progress.go

@@ -139,13 +139,6 @@ func (p *progress) warnf(format string, args ...any) {
 	}
 
 	fmt.Fprintln(os.Stderr, msg)
-
-	// The library never draws a finished bar again, so the warning for
-	// a pass's last item would erase it for good: draw its last frame
-	// again below the warning.
-	if p.bar != nil && p.bar.IsFinished() {
-		fmt.Fprint(os.Stderr, p.bar.String())
-	}
 }
 
 // finish terminates the pass's display. A bar whose pass stopped short

progress_test.go

@@ -187,25 +187,3 @@ func TestBarStoppedShortKeepsCount(t *testing.T) {
 		t.Errorf("terminal shows %q, want a count of 2 of 10", shown)
 	}
 }
-
-// TestWarningAfterLastItemKeepsBar checks that a warning printed after
-// a pass's last item, as for a failed read of the last file, leaves the
-// pass's finished bar on the terminal below the warning.
-//
-//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
-func TestWarningAfterLastItemKeepsBar(t *testing.T) {
-	stderr := captureStderr(t)
-	p := &progress{
-		label: "hash", total: 2, start: time.Now(),
-		bar: newBar("hash", 2),
-	}
-
-	p.increment()
-	p.increment()
-	p.warnf("warning")
-	p.finish()
-
-	if shown := lastFrame(stderr()); !strings.Contains(shown, "(2/2,") {
-		t.Errorf("terminal shows %q, want a count of 2 of 2", shown)
-	}
-}