Kills `PanDetector` which was the last surviving heir of the game-level
detectors. With it dead is all the infra:

* the entire `src/gestures/`, including `events.dart` with `BaseInfo`,
`PositionInfo`, `EventPosition`, `EventDelta` and all `*Info` classes
(`TapDownInfo` and `TapUpInfo` were already dead but not cleansed);
* `GestureDetectorBuilder.initializeGestures`, which existed only to
wire the detectors onto recognizers, along with the
`..initializeGestures(this)` cascade on `Game` (this will allow some
further simplifications on gesture builder binding);
* The `this is PanDetector` early-return in
`FlameGame.containsEventHandlerAt` (it was redundant, as
`componentsAtPoint` yields the game itself last, so a `FlameGame` with a
positional callbacks mixin is still reported as a hit anywhere within
its bounds);
- All the old export blocks in the `events.dart` and `input.dart`
barrels.

Old `PanDetector` users should move to `DragCallbacks` (which as usual
can be mixed into `FlameGame` directly). The examples and tutorials were
migrated separately in #4028.

Also updates and restructure the entire events docs, with this last
mercy kill:
* `gesture_input.md` is deleted (it had become a second index page
duplicating `inputs.md`, plus cross-cutting sections that I re-homed);
* the event coordinate system and the Flutter gesture-API links move to
`inputs.md`, the mouse cursor section to `pointer_events.md`, and
panning-and-zooming folds into `scale_events.md`;
* Fixed two broken links. `collision_detection.md` pointed at
`inputs/gesture_input.md#gesturehitboxes`, but that section had moved to
`inputs.md` in #4011 without its inbound links being updated;
* The drag + scale combination was documented three times, with
`drag_events.md` and `scale_events.md` carrying byte-identical code
blocks. `scale_events.md` is now canonical and `drag_events.md`
cross-links to it;
* Dropped the stale "the old (legacy) approach is described in
gesture_input.md" notes from `tap_events.md` and `pointer_events.md`.
There is no longer an old approach;
* Dropped the "For other input documents, see also:" blocks from
`keyboard_input.md` and `other_inputs.md`. They date from #2063, when
`inputs/` held exactly three documents that each linked the other two;
the gesture third has since become five pages, none of which ever got
the block. No other doc grouping in the tree links back to its index or
lists siblings (navigation is the toctree);
* Documented that `localPosition` / `localDelta` are only valid inside
the callback, with `clone()` as the escape hatch if you need the value
later.

### The Breaking

`PanDetector`, `DragDownInfo`, `DragStartInfo`, `DragUpdateInfo`,
`DragEndInfo`,
`TapDownInfo`, `TapUpInfo`, `PositionInfo`,
`GestureDetectorBuilder.initializeGestures` killed; migration guide
section is included.

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>