fix(lab): harden Drawer layer lifecycle - #6073

Why

Drawer needs to satisfy the shared overlay and native top-layer contracts while it is still in Lab, before any promotion changes are considered. Keeping this hardening separate makes the behavior review independent from the package move.

What

• Replaces Drawer’s private Escape/LIFO registry and page-level z-index ladder with the shared layer-dismissal stack.
• Gives each inactive → active cycle a fresh shared-stack identity, so a reopened sibling matches the browser top layer’s last-shown ordering without letting depth/behavior re-registration jump the queue.
• Keeps a closing Drawer registered until its visual exit finishes, so a second Escape cannot fall through to a lower layer.
• Uses showModal() for modal Drawer and a manual Popover API host for non-modal Drawer, retaining dialog.show() as the reduced fallback for browsers below the Popover API floor.
• Preserves the observable <dialog> close event for non-modal consumers after hidePopover().
• Adds the shared modal focus trap, nested layer depth, localized close label, and stable merged refs.
• Updates the current overlay/layer records, motion-lab evidence, Storybook examples, and a real-browser guard covering modal, non-modal, and stacked-exit behavior.

Drawer remains in @astryxdesign/lab in this PR. There are no Core exports, import migrations, CLI blocks, or promotion changes here.

Risk

The behavioral change is limited to Drawer lifecycle and stacking. The public props and root <dialog> element are unchanged. Non-modal Drawer remains interactive with the page behind it and retains the previous close event contract.

Testing

• pnpm lint:strict
• pnpm build
• Focused Drawer/layer/focus suites — 95 tests passed
• pnpm -F @astryxdesign/core build && pnpm -F @astryxdesign/lab typecheck
• pnpm storybook:build
• pnpm guard:modal-close --storybook-dir apps/storybook/dist
  • modal and non-modal hosts exit while rendered and hide before the next paint
  • non-modal hosts emit one close event per exit
  • stacked Drawer keeps dismissal ownership through exit
• pnpm a11y:audit -- --components Drawer — 8 stories, 0 violations
• pnpm rtl:audit -- --filter Drawer — D4 pass, no coverage gap

A package-filtered pnpm -F @astryxdesign/lab test is not a valid repo-wide invocation: it changes the process cwd to packages/lab, so root-relative CLI and fixture tests resolve paths beneath that package. The directly relevant suites above pass from the repository root.