fix(newos4): `ui` now works in classic — it was being answered by the AI (v4.6.168)

Operator report: "Ui command don't work". It was worse than a broken command: typing `ui` in the
classic terminal printed

    Security: Unknown command "ui" — routing to AI (did you mean "ai"?)...
    → Routing to AI assistant...

and an AI turn answered it. Three layers each looked healthy on its own:

1. `ui` is registered by UIShellModule, which since the v4.6.162 gate only loads when the visitor has
   already opted in — so the command that switches the interface did not exist in the state you need
   it from.
2. `_classify()` decides "known" from the command registry (module commands + a short builtin list),
   not from the safety whitelist, so `ui` was `unknown: true`. At the default security level 1 an
   unknown command that is not whitelisted is offered as a typo and then routed to the AI. `ui` was
   not whitelisted.
3. Nothing printed an error anyone would recognise as failure — it looked like the terminal was
   answering you.

Fix:
  * `'ui'` joins `_baseWhitelist` (114 -> 115). It is a navigation command: safe, non-destructive, and
    it must never depend on the interface it navigates to.
  * `dispatchCommand` gets an `if (cmd === 'ui')` fallback that mounts the shell on demand via a new
    `_mountUIShell()` (register + init, the same two steps the boot loop performs) and then opens the
    picker. It also writes `webx11_ui_experimental=1`, so the opt-in survives the reload.
  * The mount is deliberately NOT `_safeRegister('UIShellModule', …)`: that bare call is the boot gate
    and `verify-newos4-ui-gate.py` fails on any occurrence of it. A guard that grants one exception
    has to grant whatever is written next to it, so the on-demand path is separate by name.

Guard: new check `ui_command_in_classic` — executes `ui` on the served page with no flag and waits for
`webx11.uiShell` + `#wx11-ui-picker`. Deliberately behavioural: `_classify('ui').known` reads FALSE even
when the command works (registry vs whitelist), so a text or classifier assertion would have been a lie
in both directions. The RED case is on record: the AI-routing line above, captured in the live
scrollback before the fix.

Verified on v4.6.168 (served): `ui` -> picker with 7 options; `ui easy` -> easy; `ui cockpit` ->
cockpit; `ui classic` -> classic; `ui nonsense` -> "Unknown mode 'nonsense'. Available: classic, easy,
cockpit" and stays put. Guard 8/8 rc=0. maintain OK.

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