feat: make module widgets appear and respond immediately at startup

[thusser]

Implements specs/2026-08-21-gui-widget-startup-responsiveness.md.

Problem

When pyobs-gui starts (or a module connects), the nav item appears before the widget behind it is ready: clicks during open() are silently dropped (_change_page early-returns for unregistered clients), the page shows a blank/disabled widget until state arrives, and startup itself is serialized per module — with the telescope's heavy open() chain blocking everything else and the per-connect Shell command-model rebuild being O(N²) with synchronous interface introspection on the UI thread.

Changes

• mainwindow.py — placeholder + background open. _add_client registers the widget, a clickable "Loading…" placeholder page, and a background open() task the moment a module connects; _change_page/shortcuts always find the client, so the click is never dropped. When open() finishes, the real widget replaces the placeholder at the same stack index (and becomes current + _init() fires if the user was sitting on that page). A failing open() tears the client down (_fail_open) instead of leaving a permanent dead page; a mid-open disconnect cancels and awaits the open before discarding, and removes the placeholder from the stack (no ghost page); discard_all_widgets() drains pending opens before teardown.
• mainwindow.py — parallel startup. _init_clients uses asyncio.gather; the per-connect _update_client_list/_check_warnings global work is dropped (menu rebuild kept as cheap sync _update_clients_menu, warnings covered by the periodic task + one post-gather pass).
• base.py — memoized _init. _showEvent runs _init() through a memoized task so two rapid show/hide/show cycles can't double-subscribe; a failing _init() leaves the widget un-initialized so the next show retries.
• shellwidget.py — debounced, visibility-gated command model. The Shell rebuilds its own CommandModel on module events, debounced (0.5 s) and only while its page is visible; a first-show hook rebuilds lazily so a never-opened page isn't left with an empty completer. (Required for the O(N²) startup-cost fix, since the mainwindow no longer triggers the rebuild per connect.)
• telescopewidget.py — _init() fires the five subscribe_state calls concurrently.
• camerawidget.py — _init() runs each interface's caps → state → subscribe chain concurrently (per-interface ordering preserved) and wait_for_state uses a 2 s timeout instead of the 10 s default, so a slow-publishing camera can't hold the page blank for ~70 s.

Tests

New tests/test_mainwindow_startup.py (9 tests, offscreen Qt): placeholder-then-swap, open-failure teardown, mid-open disconnect without ghost page, discard_all_widgets drain, parallel _init_clients, telescope parallel subscriptions, _showEvent single-init + retry-on-failure, and the shell visibility gate / lazy rebuild. Full suite: 41 passed.

Black, ruff, and pyrefly are clean.

Out of scope (see spec)

Pre-warming _init() at open time, moving CommandModel.init's introspection off the UI thread, StatusWidget's own per-module RPC chains, and the pre-existing ShellWidget event-handler registration leak.