feat: useMenu hook — active branch, disclosure state, breadcrumb trail

[dennation]

The top layer of the active-menu work: a hook that owns disclosure state and derives the active branch, plus the two things it stands on.

Three layers, each reusable on its own:

Core (React-free) — @dennation/menu

• getMenuTrail(menu, id) — the ancestor chain down to id (breadcrumbs and the branch to expand). Usable on the server / in loaders, no React.
• findMenuItem(menu, id) — the node, sharing that traversal.

<Menu> learns controlled state — @dennation/menu/react

• open / onOpenChange to control disclosure from outside; defaultOpen to seed the uncontrolled case.
• State is a sparse Record<id, boolean> of overrides, not a list of open sections — so item.defaultOpen still wins by default and the common case stays empty. (This is the one place I stepped off the earlier openIds: string[] sketch — a list would be near-complete under "open by default"; a map of overrides is smaller and gives one isOpen in both modes.)

useMenu — @dennation/menu/react

const { menuProps, trail, activeItem, open, close, toggle, isOpen } =
  useMenu(menu, { activeId: pathname });
<Menu {...menuProps} components={components} />;

Auto-expands the active branch on activeId change, exposes trail/activeItem for breadcrumbs, and open/close/toggle/isOpen for imperative control — RHF-style bind-props (Variant A), not a returned component.

Scope I deliberately trimmed

• filterMenu / flattenMenu (rights, ⌘K, prev/next) — left out; no consumer in this PR. Add when one lands.
• External-store controlled useMenu — the hook owns state; a fully-controlled openIds input can come later if needed.

Test plan

• pnpm test — 35 passed (was 24): trail/find, controlled <Menu>, useMenu auto-expand + imperative toggles
• pnpm typecheck — clean
• pnpm build — useMenu/getMenuTrail/findMenuItem in declarations
• biome check src — clean

Branched from main; independent of #3 (touches no adapter files).