opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

apps/desktop/electron/bootstrap-runner.cjs

@@ -40,15 +40,6 @@ const path = require('node:path')
 const https = require('node:https')
 const { spawn } = require('node:child_process')
 
-const IS_WINDOWS = process.platform === 'win32'
-
-function hiddenWindowsChildOptions(options = {}) {
-  if (!IS_WINDOWS || Object.prototype.hasOwnProperty.call(options, 'windowsHide')) {
-    return options
-  }
-  return { ...options, windowsHide: true }
-}
-
 const STAMP_COMMIT_RE = /^[0-9a-f]{7,40}$/i
 
 // Stages flagged needs_user_input=true in the manifest are skipped by the
@@ -293,7 +284,7 @@ function spawnPowerShell(scriptPath, args, { emit, stageName, abortSignal, herme
     const ps = process.platform === 'win32' ? resolveWindowsPowerShell() : 'pwsh'
     const fullArgs = ['-NoProfile', '-ExecutionPolicy', 'Bypass', '-File', scriptPath, ...args]
 
-    const child = spawn(ps, fullArgs, hiddenWindowsChildOptions({
+    const child = spawn(ps, fullArgs, {
       stdio: ['ignore', 'pipe', 'pipe'],
       env: {
         ...process.env,
@@ -301,7 +292,7 @@ function spawnPowerShell(scriptPath, args, { emit, stageName, abortSignal, herme
         // choice rather than re-computing the default.
         HERMES_HOME: hermesHome || process.env.HERMES_HOME || ''
       }
-    }))
+    })
 
     let stdout = ''
     let stderr = ''