Audit what a profile saves, and close the two gaps it found

Every setting was checked by setting all of them to something other than
their default, saving, loading, and comparing: all 54 survived, ranges kept
every field of their own, and the default settings file behaved the same as
a named profile. Those checks are now tests rather than a one-off, along
with a check that no setting is saved but never read, and that a profile
written by an older version still loads.

Two gaps were real.

The plain display could only be asked for per invocation. It was a
hand-written flag rather than an entry in the settings table, so unlike
`quiet` -- which does the same sort of thing -- it could not be saved,
leaving anyone who runs over ssh to pass --plain every time. It is a
setting now, which gives it a menu entry and --no-plain for free.

Gain displayed as "28.5 dB" but refused to parse "28.5 dB" back, because it
was the one numeric setting that called float() instead of the unit-aware
parser every other one uses. The menu shows a value as the example of how
to type it, so what it shows has to be something it accepts.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

bandsaunter/cli.py

@@ -78,8 +78,6 @@ examples:
     st.add_arguments(s)
 
     g = s.add_argument_group("presentation")
-    g.add_argument("--plain", action="store_true",
-                   help="line-per-hit output instead of the live display")
     g.add_argument("--simulate", action="store_true",
                    help="use a synthetic receiver instead of real hardware")
     g.add_argument("--dry-run", action="store_true",
@@ -336,7 +334,7 @@ def cmd_scan(args) -> int:
         return 2
 
     signal.signal(signal.SIGINT, lambda *a: scanner.stop())
-    rc = (_run_plain(scanner, cfg) if (args.plain or cfg.quiet or
+    rc = (_run_plain(scanner, cfg) if (cfg.plain or cfg.quiet or
                                        not sys.stdout.isatty())
           else _run_live(scanner, cfg))
     _print_summary(scanner)