Add bounded parallel preview rendering

[static-var]

Summary

• add maxParallelRenders DSL property and CLI override
• default remains 1 to preserve existing sequential behavior
• execute planned captures with bounded parallelism using a fixed thread pool
• keep dry-run/maxCaptures checks before rendering
• preserve continueOnError aggregation and fail-fast behavior
• include maxParallelRenders in capture reports

Behavior

• -PagentPreview.maxParallelRenders=2 enables two concurrent render jobs
• invalid, zero, and negative values fail with actionable errors
• fail-fast mode stops scheduling new captures after first observed failure; already-started captures may finish
• continueOnError=true continues through all planned captures and reports failures

Verification

• build-brief ./gradlew spotlessCheck detekt :plugin:test :preview-scanner:test
• ANDROID_HOME=$HOME/Library/Android/sdk build-brief ./gradlew -p samples/android-compose-app :app:captureComposePreviews --configuration-cache
• ANDROID_HOME=$HOME/Library/Android/sdk build-brief ./gradlew -p samples/cmp-compose-app :composeApp:captureComposePreviews --configuration-cache

VLR smoke

In /Users/staticvar/Projects/vlr-gg:

ANDROID_HOME=$HOME/Library/Android/sdk build-brief --mode raw ./gradlew :designsystem:captureComposePreviews -PagentPreview.maxParallelRenders=2 -PagentPreview.previewNameFilter=PrismButtonPreview,PrismCardDarkPreview -PagentPreview.maxPreviewParameterValues=1 -PagentPreview.viewportFilter=phone --configuration-cache --console=plain

Result: BUILD SUCCESSFUL.

Review

• Implementation delegated to subagent
• Separate review subagent approved with no Critical/Major/Minor findings