Add hit-test position selector

[passsy]

Adds hit-test based position selectors for widgets at a global screen coordinate.

spotAtPosition(const Offset(400, 300)).existsAtLeastNTimes(1);
spot<ColoredBox>().atPosition(const Offset(400, 300)).existsOnce();

spotAtPosition starts a selector from a global position.
atPosition narrows an existing selector by the Flutter hit-test path.
The selectors follow Flutter hit testing, so overlapping siblings that do not receive the hit are not reported.

Fixes #28

[Copilot]

Pull Request Overview

This PR adds utilities to select multiple widgets at a given screen position.

• Introduce a PositionFilter and PositionSelectors extension to hit-test widgets by global coordinates
• Remove the old singular selector and expose the new spotWidgetsAtPosition in the public API
• Update tests and changelog to reflect the new plural selector

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/spot/widget_at_position_test.dart	Adapted tests to use spotWidgetsAtPosition and added new cases
lib/src/spot/selectors.dart	Imported PositionFilter and added PositionSelectors extension
lib/src/spot/filters/position_filter.dart	Implemented widget hit-testing logic via PositionFilter
lib/spot.dart	Exposed top-level spotWidgetsAtPosition and exported PositionFilter
CHANGELOG.md	Bumped to 0.18.1 and documented the new selector

Comments suppressed due to low confidence (1)

test/spot/widget_at_position_test.dart:49

• [nitpick] Consider adding a test case where spotWidgetsAtPosition is called at a point with no widgets (e.g., outside the stack) to ensure it returns an empty result.

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