feat: inline (non-modal, non-top-layer) mode for overlays, for theme previews and showcases #563

Theme previews and showcases need to render the real overlays inside a themed island. Today they cannot: a modal Dialog is moved to <body> and shown with showModal(), and menus, popovers and tooltips use popover="manual" + showPopover(). Both go to the browser top layer, so they leave the island and lose its data-preset / data-theme-scope tokens. The Theme Studio (#560) therefore previews overlays as static copies.

Ask

An opt-in inline mode: the overlay renders where it is declared, in normal flow or absolutely positioned inside a position: relative host, never in the top layer.

• Dialog, Sheet, Drawer, AlertDialog: inline renders the panel as a non-modal <dialog open> (show()), with a faux scrim drawn by the host. No inert page, scroll lock or focus trap.
• FloatingLayer (so DropdownMenu, Popover, Tooltip, HoverCard, ContextMenu): inline skips popover/showPopover(), and positions against its anchor with position: absolute inside the host.
• Toaster: inline positions absolute in its container instead of fixed.
• Shared plumbing in primitives/layer.ts: an inline layer option that does not push onto the global stack (no Escape or outside-press ownership), so an inline overlay can never steal input from a real one.

Acceptance

• Test first: a harness renders each overlay inline inside an element with data-preset="violet" and asserts the overlay's computed --ca-brand-main is the island's, not the page's. It fails on current code.
• A real modal opened on the same page still owns Escape and inertness while inline overlays are present.
• Docs section on theme previews, docs/css-api.md updated, axe clean in both modes.

Status

Blocks "tweakcn parity done". The Theme Studio (#560, PR #562) previews overlays as static copies, labelled as such in the UI. Real Dialog, Menu and Toast inside a themed island is the last gap against the owner's bar, so parity is not done until this lands.

Verification

This changes the shared layer engine (primitives/layer.ts: the stack, Escape and outside-press ownership, modal inertness, focus trap, scroll lock, positioning). It must pass the full overlay and layer suites (primitives/layer, Dialog, Sheet, Drawer, AlertDialog, Popover, DropdownMenu, Tooltip, HoverCard, ContextMenu, Toaster, nested layers) and the route sweep (routes.a11y, overlays.a11y, dialog-modal, scroll-lock), not only the studio tests.