test(ensemble): cover Page header listeners and ListViewCore scroll rebind

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Summary

Adds regression tests for two recent production fixes that shipped without full test coverage.

Risky behavior now covered

• Page header storage listeners (2c45e427): PageState previously registered three duplicate ModelChangeEvent listeners for titleBarHeight storage updates and never cancelled periodic poll timers on dispose. Tests assert a single stored subscription is created and that poll timers are cancelled cleanly when the page is removed.
• ListViewCore scroll controller sync (c792a8a6): When a caller-provided ScrollController is swapped at runtime, ListViewCore must rebind its internal Scrollable (not only the public ListViewController). A widget test verifies the Scrollable.controller tracks the new external controller.

Test files added/updated

• modules/ensemble/test/widget/page_header_storage_listeners_test.dart (new)
• modules/ensemble/lib/framework/view/page.dart — @visibleForTesting getters for listener/timer state (no behavior change)
• modules/ensemble/test/widget/list_view_test.dart — external scroll controller swap case

Why these tests materially reduce regression risk

• Header storage listeners run on every page with dynamic AppBar height; duplicate subscriptions multiplied rebuild work and undisposed timers could leak callbacks across navigations.
• Scroll controller desync between ListViewController and ListViewCore breaks programmatic scroll APIs and footer handoff; the new test complements the existing footer-scope regression test.

Validation

cd modules/ensemble
flutter test test/widget/page_header_storage_listeners_test.dart
flutter test test/widget/list_view_test.dart --plain-name "ListViewCore rebinds when external scroll controller changes"

Both new tests pass. Note: the existing footer-scope ListView test appears flaky in this VM environment (also fails on main); not introduced by this PR.