Multi-playing: recoverable loadProfile when the browser blocks it, honest setActiveProfile across tabs

[Delwing]

Addresses #453. This PR doesn't close it; the three remaining gaps are under "Not fixed here" below.

Changes

• loadProfile(): the browser blocks the new tab on a second call in one alias, and on any call from a trigger, timer or event.
  • In that case it returns nil, msg and leaves an "Open profile " link on its own line in the main console.
  • Clicking the link opens and connects the profile, because the click is the user gesture the browser requires.
  • If the profile is already open by then, or the click is also blocked, the link says so instead of opening a second tab.
• setActiveProfile(): for a profile open in another tab it now returns false, msg instead of a false true, since a page can't bring another tab to the front.
  • The other tab gets a new attention message over the profiles-presence BroadcastChannel and flashes its title until the user switches to it.
  • sysProfileFocusChangeEvent fires when they do.
  • Calling it for the current tab's own profile still returns true.
• raiseGlobalEvent(): behaviour is unchanged.
  • There's no avoidable delay: it sends synchronously, and the receiving tab hands the event straight to the Lua runtime.
  • A comment guards against anyone adding a delay later.
  • Tests confirm the event arrives after the call returns and never reaches the sender, matching desktop's postInterHostEvent.
• Documentation: all three differences from desktop are recorded in e2e/knownDivergences.ts, docs/MUDLET_API.md and luaCompletions.ts.
• Tests: tests/scripting/multiProfile453.test.ts.

Not fixed here

These remain, and all of them come from running each profile in its own tab:

• Several alts from one alias: browsers allow one new tab per click or key press, so the second and later alts each need a click on their link.
• setActiveProfile() across tabs can't actually switch to the other tab.
• raiseGlobalEvent() replies arrive one task later.

Closing these needs several profiles hosted in one tab, as desktop does. That's an estimated 35–50 file change and is out of scope for this PR.

Testing

• yarn typecheck passes.
• yarn test: 3 failures, none caused by this branch:
  • serverEncoding.test.ts (GBK euro sign) also fails on master.
  • added-apis (clipboard) and mapperParity295 fail only in the full parallel run and pass when run alone.
• Not tried in a real browser with multiple tabs.

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