[cupertino_ui] Reuse a single Paint in _CupertinoEdgeShadowPainter

[fa0311]

The edge shadow of a Cupertino page transition is drawn as ~20 1px-wide rects per frame, each allocating a fresh Paint. This runs on every frame of every CupertinoPageRoute push/pop and back-swipe. This PR hoists the Paint out of the loop and only updates its color per iteration, matching the pattern already used by _CupertinoActivityIndicatorPainter. Canvas draw calls snapshot paint state, so mutating and reusing a single Paint produces identical output.

Fixes flutter/flutter#190198. (Originally submitted to flutter/flutter as flutter/flutter#190188, redirected here due to the Material/Cupertino decoupling code freeze, flutter/flutter#184093.)

This is a behavior-preserving refactor with no observable output change; existing tests in packages/cupertino_ui/test/route_test.dart cover this code path and pass, so I believe this qualifies as test-exempt.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request optimizes the _CupertinoEdgeShadowPainter in route.dart by instantiating a single Paint object outside the loop and reusing it, rather than allocating a new object for each iteration. Additionally, the package version is bumped to 0.0.3 in pubspec.yaml and the change is documented in CHANGELOG.md. There are no review comments, and I have no feedback to provide.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. If you believe this PR qualifies for a test exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!). The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[Piinks]

Hi @fa0311 thank you for the contribution. I am going to close this for now because we are not accepting PRs in these packages currently. We are still in the middle of the migration and ensuring the packages are complete before publishing and accepting contributions. Thank you.

If you would like to re-open the PR you had in flutter/flutter, we'll be helping to port these PRs over when we are ready to start accepting contributions.