testing: cover modal, dialog, select, and dropdown behavior

[RtlZeroMemory]

Summary

• cover app-level modal, dialog, select, and dropdown behavior in one reviewable landing PR after the earlier stacked branch was collapsed
• cover dialog escape close with focus restore and stacked modal focus restoration through useModalStack(...)
• cover select keyboard progression with disabled-option skipping and dropdown outside-click close while retaining existing item-click coverage

Families Covered

• modal
• dialog
• useModalStack
• select
• dropdown

Tests Added, Rewritten, Removed

• added app-level tests in packages/core/src/app/__tests__/widgetBehavior.contracts.test.ts for dialog escape close + focus restore, stacked modal focus restoration, select keyboard progression, and dropdown outside-click close
• rewrote no existing tests in this PR
• removed no tests in this PR because the retained lower-level modal, hook, shared-scenario, and dropdown routing suites still protect distinct contracts

Implementation Bugs Fixed

• none; the behavior-first tests passed once added

Commands Run

• ./node_modules/.bin/tsc -b packages/core/tsconfig.json --pretty false
• node --test packages/core/dist/app/__tests__/widgetBehavior.contracts.test.js packages/core/dist/app/__tests__/widgetRenderer.integration.test.js packages/core/dist/testing/__tests__/referenceScenario.semantic.test.js packages/core/dist/widgets/__tests__/modalStack.hook.test.js packages/core/dist/widgets/__tests__/modal.focus.test.js packages/core/dist/widgets/__tests__/dropdown.position.test.js packages/core/dist/runtime/__tests__/overlayUnification.test.js

Unresolved Areas Left Explicit

• broader mixed overlay pointer ownership remains separate from the focus/close paths covered here
• dropdown shortcut behavior and long-menu visible-window behavior remain protected by retained lower-level tests rather than new app-level assertions
• tree lazy-load failure presentation remains under-specified and was not forced into this PR

Dependency Note

• none; #369 is already merged and this branch already absorbed the previously stacked select/dropdown slice

#Closes 346
#Closes 345

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