Fix event filtering on tags

[edalzell]

Problem

The event parameter on the events tags (between, upcoming, today, in, calendar) is ignored. Passing event="some-id" returns all occurrences in the collection instead of only the occurrences for that single entry.

Fixes #173.

Reason

In the v6 refactor (#168), Events::entries() was rewritten to delegate to a new Entries query class that only filters by collection:

$query = Entry::query()->whereIn('collection', $this->collections->map->handle()->all());

The previous implementation branched on the event id to filter where('id', $id). That branch was dropped — the generator still stores the id in $this->event, but the field became dead code and is never read by the query. So every tag routed through Entries returns the whole collection regardless of event.

Solution

• Restore the id filter in src/Entries.php using a two-branch when() so event and collection are mutually exclusive, with event taking precedence — matching the v5 behavior.
• Guard against ambiguous usage: the Events generator constructor now throws if both collection and event are passed. The check runs on the raw params before collection gets its 'events' default applied, so it reflects true user intent. This does not affect events:download_link, which legitimately accepts both.
• Tests: reproduce the regression (a specific event now returns 4 occurrences, not 5 when an unrelated entry exists in the collection) and cover the new exception.
• Docs: note that collection and event are mutually exclusive across the query tags.