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>

lua/turbo-debug/config.lua

@@ -1,7 +1,9 @@
 local M = {}
 
 M.defaults = {
-  help_on_enter = true,
+  -- The 3-row control bar shows `(?)help` so users can always see where to
+  -- get the key reference. No auto-popup needed on entry.
+  help_on_enter = false,
   builtin_adapters = true,
 
   -- When `c` is pressed with no breakpoints set, launch the first adapter