feat: 3-row control bar with key hints, box lines, help in bar

UX refinement per design feedback:

- Bar is now 3 rows tall: ─ top separator, content row, ─ bottom separator.
  The separator chars were invisible before because TurboDebugBarSeparator
  linked to WinSeparator (muted in most themes). Switched to FloatBorder.
- Bottom tray default shrunk 12→9 rows so the 3-row bar doesn't overlap
  REPL/Console. Net vertical footprint unchanged (12 tray = 9 tray + 3 bar).
- Action labels now teach the keyboard shortcut inline:
    ▶️ (c)ontinue   ⏭️ (s)tep   ⬇️ (d)escend   ⬆️ (r)eturn
    🔄 (R)estart   🛑 (q)uit   ❓ (?)help
  The (k) letter is highlighted with TurboDebugBarKey → Special so the
  shortcut pops. The entire label is a click zone.
- help_on_enter default flipped true → false. The (?)help label in the bar
  is always visible, so new users discover help via the bar instead of an
  auto-popup on every debug-mode entry.
- No top bar added. The source file winbar already shows 🐛 DEBUG · <file>,
  so a second chrome row would just duplicate context and eat screen space.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_config.lua

@@ -7,7 +7,7 @@ assert_eq(opts.keys.continue, "c", "default continue key")
 assert_eq(opts.keys.breakpoint, "x", "default modal breakpoint key")
 assert_eq(opts.global_keys.toggle, "<leader>dd", "default toggle key")
 assert_eq(opts.global_keys.breakpoint, "<leader>dx", "default global breakpoint key")
-assert_eq(opts.help_on_enter, true, "default help_on_enter")
+assert_eq(opts.help_on_enter, false, "default help_on_enter")
 assert_eq(opts.stop_on_entry_when_no_breakpoints, true, "default stop_on_entry_when_no_breakpoints")
 assert_eq(opts.max_value_width, 2000, "default max_value_width")
 assert_eq(opts.console_refresh_ms, 50, "default console_refresh_ms")