[Modal] Dynamically wire unique aria-labelledby and remove hardcoded IDs

DESIGN.md

@@ -529,7 +529,7 @@ Component-specific behaviors are never promoted to Canonical Design Rules withou
   - **Enforcement Status**: Contrast is guided by theme tokens and helper utilities (`readableTextColor`), but is not enforced by a global automated linting/test suite across every component.
 - **Label Capitalization**: Action labels use `textTransform: 'capitalize'` built into `textB2SemiBold`.
 - **Semantic ARIA**: Icon-only buttons must supply descriptive `aria-label` and `Tooltip`.
-- **Modal Dialog Accessibility (`aria-labelledby`)**: Sistent's `Modal` component (`src/custom/Modal/index.tsx`) hardcodes `aria-labelledby="alert-dialog-slide-title"` and `aria-describedby="alert-dialog-slide-description"` on the underlying dialog, while its internal title element does not currently forward an `id`. This library-level accessibility gap is tracked in [#1864](https://github.com/layer5io/sistent/issues/1864). As a temporary consumer workaround until resolved in the library, consumers should conditionally mount dialog instances (`{isOpen && <Modal ... />}`) rather than keeping multiple hidden instances mounted simultaneously to avoid duplicate-ID collisions in the DOM accessibility tree.
+- **Modal Dialog Accessibility (`aria-labelledby` / `aria-describedby`)**: Sistent's `Modal` component (`src/custom/Modal/index.tsx`) dynamically generates unique title IDs via MUI's `unstable_useId` wired to `aria-labelledby` on the dialog paper, eliminating hardcoded IDs and DOM ID collisions across concurrent modals (#1864). `ModalProps` enforces an accessible name at compile time via a union requiring `title`, `aria-label`, or `aria-labelledby`. When a visible title is non-empty, `aria-labelledby` defaults to the generated title element ID; when blank or whitespace-only, the visible title header is omitted. Titleless modals are supported via explicit `aria-label` or `aria-labelledby` props on `Modal` or through `slotProps.paper`. Caller-provided accessibility attributes are attached directly to the dialog paper (kept off the presentation dialog root). In accordance with the W3C Accessible Name (accname) specification, `aria-labelledby` takes precedence over `aria-label`, and Modal-level naming props take precedence over `slotProps.paper` attributes. Both object and callback forms of `slotProps.paper` are preserved without mutating caller objects.
 
 ---