fix: eliminate popover flash and stage cutout jump during anchorless step transitions

[saikat709]

Fixes #549

Summary of Changes

When navigating between consecutive tour steps without an attached element (anchorless/modal steps), a white square (.driver-popover) flashed at the CSS default position (top: 0, right: 0), and the overlay stage cutout box briefly jumped/slid towards (0, 0).

This PR fixes the issue across four key areas:

1. Popover Visibility Guard (packages/driver/src/popover.ts):

   • Set visibility: hidden on popoverWrapper prior to resetting position attributes (left, top, right, bottom) in renderPopover, restoring visibility: "" only after repositionPopover() completes positioning. This eliminates unpositioned popover flashes.

2. Step Transition State (packages/driver/src/highlight.ts):

   • Refactored isFirstHighlight calculation: !fromStep || !fromElement || (fromElement === toElement && !isToDummyElement).
   • Ensures transitions between consecutive #driver-dummy-element steps preserve hasDelayedPopover = true, enabling smooth popover content reuse without unnecessary teardown gaps.

3. In-Place Stage Animation (packages/driver/src/overlay.ts):

   • Refactored transitionStage() using a centerOf() helper:
     • Anchored $\rightarrow$ Anchorless: Collapses cutout in-place at the originating element's center.
     • Anchorless $\rightarrow$ Anchored: Expands cutout outward from the destination element's center.
     • Anchorless $\rightarrow$ Anchorless: Renders a zero-cutout full-screen overlay.

4. Zero-Cutout SVG Path Guard (packages/driver/src/stage.ts):

   • Added an isDummy check in generateStageSvgPathString() so dummy element stages render a full-screen viewport path without cutout subpaths.

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Verification & Testing

• Added comprehensive unit tests in highlight.test.ts, overlay.test.ts, and stage.test.ts.
• 100% of all 23 unit test files (338 tests) passed cleanly (pnpm --dir packages/driver test:run).
• Verified in the playground app (pnpm dev) across both fast (150ms), default (400ms), and slow (1500ms) anchorless step transitions.