fix(desktop): resync device scale after display changes

[besmpl]

Summary

• synchronize the UI canvas device scale from the current gogpu frame before the idle-render gate
• invalidate scale-dependent retained scenes, repaint boundaries, overlays, textures, and layer state when the scale changes
• add focused regressions for scale-only display transitions in both the app and desktop layers

Why

A window moving between displays with different scale factors can retain its logical size. In that case the resize path does not run, and RenderDirectWithDamageRects bypasses ggcanvas's later provider-scale check. The old device scale, boundary textures, and retained scene content can therefore survive the transition.

The new frame-level synchronization updates scale-dependent state before deciding that a frame is idle, ensuring the scale-change redraw produces correctly rasterized content.

Verification

• go test ./... -count=1
• go build ./...
• CGO_ENABLED=0 go build ./...
• focused app/desktop scale tests, including repeated and shuffled runs
• focused race tests
• go vet ./...
• git diff --check

The broad go test -race ./app ./desktop run still reports the repository's pre-existing mockWindowProvider.RequestRedraw race in TestWindow_AnimPumper_StartsOnInvalidation; the changed desktop tests pass under -race.

Fixes #172

Local CI and Codecov preflight

• rebased onto current main (273cc1e)
• go test ./... -count=1 with an atomic coverage profile: pass
• focused changed-behavior race suites: pass; the broad app/desktop race remains blocked only by the pre-existing TestWindow_AnimPumper_StartsOnInvalidation mock race noted above
• go build ./..., go vet ./..., gofmt, and diff checks: pass
• local Codecov-equivalent changed-production coverage: 30/30 statements (100%) (app/window.go 21/21, desktop/desktop.go 9/9); overall coverage 86.4%

Upstream CI run approval is still external to this branch: GitHub marked the fork workflow action_required with zero jobs (run 31434862113). A gogpu maintainer must approve the workflow before Actions and the configured Codecov upload/bot can run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[kolkov]

APPROVE

Well-structured two-layer fix:

1. desktop/desktop.go:syncDeviceScale -- frame-level sync placed correctly before the O(1) idle gate and before boundary texture allocation. Releases boundary textures, resets canvas scale, delegates to Window.HandleScaleChange.

2. app/window.go:HandleScaleChange + invalidateScenesInTree -- widget-level invalidation covering ADR-024 boundaries, legacy RepaintBoundaryMarker, and overlay content widgets.

Test coverage is thorough: 6 tests covering boundary invalidation (including the needsRedraw+cleanScene edge case), nil root safety, non-positive scale rejection, texture release + layer tree reset, and integration-level placement in the render loop.

The reflect+unsafe usage in device_scale_sync_test.go is consistent with the existing pattern in gpu_work_test.go -- acceptable for test code that has no alternative since gogpu.Context is only fully populated during a live frame.

CI green (9/9 checks pass). No blocking issues.