Print progress at once off a terminal, keep stderr to one writer (closes #13)

On a pipe, a file or /dev/null each scan phase now prints its zero-state
line the moment it starts instead of after its first finished item, so
a slow first read under cron no longer leaves the log silent. Terminal
detection uses golang.org/x/term, now a direct dependency listed in the
README; the old character-device test took /dev/null for a terminal.
The walk spinner no longer redraws from the library's own goroutine; it
advances only as items complete, so a redraw cannot garble a warning.
Building the bar moved into newBar so the terminal display can be
tested without a terminal.

Lint suppressed: gosec on two test file opens, paralleltest on three
tests that replace os.Stderr.

Model: opus-5-5

README.md

@@ -113,7 +113,8 @@ Goals, in order:
     `sfdupes`.
 -   Dependencies: standard library, `github.com/spf13/cobra` for the
     CLI, **one progress-bar library**
-    (`github.com/schollz/progressbar/v3`), and **one SQLite driver**
+    (`github.com/schollz/progressbar/v3`), `golang.org/x/term` to tell
+    whether stderr is a terminal, and **one SQLite driver**
     (`modernc.org/sqlite`, pure Go, so builds keep cgo disabled).
     `github.com/spf13/viper` is permitted if configuration-file support
     is ever needed, but is not currently used. No other third-party
@@ -533,10 +534,13 @@ hash: [12345/98765] 12% |████      | 92 files/s elapsed 2:32 eta 17:54
 
 Additional requirements:
 
--   When stderr is not a TTY, do not emit ANSI redraws: print a plain
-    one-line progress update no more often than every 5 seconds instead.
+-   When stderr is not a terminal (a pipe, a file, `/dev/null`), do not
+    emit ANSI redraws: print a plain one-line progress update the moment
+    each phase starts, then no more often than every 5 seconds.
 -   Progress updates are driven from the main goroutine and must be
-    non-blocking with respect to the worker pool.
+    non-blocking with respect to the worker pool. Nothing redraws in the
+    background (the spinner advances only as items complete), so a
+    warning printed during a phase always lands on its own line.
 -   `report` and `trees` modes need no progress display, only their
     stderr summaries.