Migrate GridThumbDragDevIndex whitebox-e2e onto the locked-column fixture (split scroll-telemetry vs locked-cell)

Context

test/playwright/e2e/GridThumbDragDevIndex.spec.mjs was skipped when the devindex locked-column config was removed, and was deferred from the #12945 / #12936 fixture migration (which re-homed GridColumnCrossBodyDnD + GridColumnOverdragScroll onto the new examples/grid/lockedColumns fixture).

The Problem

GridThumbDragDevIndex is a mixed spec — its two tests have different needs:

• Horizontal drag / locked-cell stability — queries .neo-locked-start / .neo-locked-end cells and asserts they stay visually frozen during horizontal scroll. A locked-column concern that fits the examples/grid/lockedColumns fixture.
• Vertical scroll telemetry / row-pinning — Profiles 1-7 (wheel + synthetic + thumb drags) asserting zero blank-frames / jitter. A vertical-virtualization concern that needs a many-row streaming grid, not a small locked-column fixture; its beforeEach also depends on devindex's stream-stop button.

Retargeting cleanly means splitting these two concerns — real per-spec work distinct from standing up the locked-column fixture itself, which is why it was deferred.

Acceptance Criteria

• Split the locked-cell horizontal-stability test out and re-home it onto examples/grid/lockedColumns (un-skip it).
• Re-home or re-scope the vertical scroll-telemetry / row-pinning test onto a many-row grid that survives the devindex lock removal (devindex post-removal or examples/grid/bigData), independent of locked columns.
• Remove the ticket-ref-ok skip annotation from the spec once both land.

Related

#12936 / #12945 (the fixture this builds on); #12933 / #12935 (devindex lock removal — the forcing event).