Add an opt-in framework hit-test region pass for platform views - #195607

This adds an experimental, opt-in framework pass that computes the regions where Flutter content painted on top of a platform view would claim pointer events. A later change will have the web engine use these regions to place hit-test proxies, so content drawn over HtmlElementViews gets pointer events without wrapping it in a PointerInterceptor.

Behind the new RendererBinding.automaticPointerInterceptorsEnabled flag (off by default; it only has an effect on the web unless debugEnableHitTestRegionsOnAllPlatforms is set), PipelineOwner.flushHitTestRegions() runs after compositing each frame:

• PlatformViewRenderBox reports attach/detach to its PipelineOwner, so the walk is skipped entirely when an owner has no platform views attached.
• A walk (in child order; paint-order overrides follow in a later PR) tracks the global transform and ancestor paint clips. It groups the physical-pixel rects of hit-testable boxes painted after each platform view, and keeps only rects that overlap an earlier platform view. Offstage subtrees are skipped. Translucent targets don't claim a rect.
• Render objects describe whether pointer events can reach their children through a new protected getter, RenderObject.hitTestsChildren (default true). RenderIgnorePointer returns !ignoring and RenderAbsorbPointer returns !absorbing. The walk reads the getter instead of checking for specific render object types, so third-party render objects that block hit testing of their children can take part by overriding it. When it is false, no descendant claims a rect, but platform views in the subtree are still found and start their own groups (the browser still sends them pointer events). The render object's own claim isn't affected, so an absorbing AbsorbPointer still claims its own bounds.
• The result is sent as a flat Float64List ([viewId, n, l, t, r, b * n, ...]) to a new PipelineOwner.onHitTestRegionsUpdate callback. It's also exposed through debugLastHitTestRegions for tests. When the last platform view goes away or the flag is turned off, an empty payload is sent once.
• The pass recurses through child pipeline owners, so it works with multiple views.

Part of #179806

Behavioral Changes

• With automaticPointerInterceptorsEnabled off (the default), nothing changes at runtime. The only always-on addition is the attach/detach counter on PipelineOwner for platform views.
• New API: RendererBinding.automaticPointerInterceptorsEnabled, PipelineOwner.flushHitTestRegions, PipelineOwner.onHitTestRegionsUpdate, PipelineOwner.debugLastHitTestRegions, debugEnableHitTestRegionsOnAllPlatforms, and the protected RenderObject.hitTestsChildren getter (overridden by RenderIgnorePointer and RenderAbsorbPointer). The getter doesn't change hit testing. Only the new pass reads it.

Where to focus

• RenderObject.hitTestsChildren. It's a new protected member on RenderObject. It sits on RenderObject rather than RenderBox so that sliver render objects (for example RenderSliverIgnorePointer, in the follow-up paint-order PR) can override it too. Its contract is that it must agree with the render object's own hitTest/hitTestChildren.
• Remaining type checks. The walk still has two is checks: RenderOffstage (skips the subtree) and an absorbing RenderAbsorbPointer claiming its own bounds. Follow-up PRs replace them with visitChildrenInPaintOrder and a region override point.
• Child order, not paint order. The walk follows visitChildren and treats anything visited after a platform view as painting over it. That matches paint order for most render objects, but not for viewports that paint slivers in reverse, or for IndexedStack. Those cases are left for the follow-up paint-order PR.
• Frame hook. The pass runs from its own persistent frame callback registered in RendererBinding.initInstances, not from drawFrame, because the test bindings override drawFrame without calling super.
• Empty-payload teardown. One empty payload is sent when the last platform view detaches or the flag is turned off, and only on platforms where the pass runs.
• Primary files: packages/flutter/lib/src/rendering/hit_test_regions.dart (the walk and encoding), then object.dart (PipelineOwner bookkeeping and hitTestsChildren), proxy_box.dart (the IgnorePointer/AbsorbPointer overrides) and binding.dart (flag and frame callback).

Pre-launch 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 wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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