feat(core): add SidebarLayout body slot for custom page layouts

[interacsean]

What

Adds an eject seam for everything to the right of the sidebar, so consumers can arrange their own columns — a table-of-contents rail, an edge-docked assistant panel — without overriding AppShell internals.

Addresses tailor-inc/platform-planning#1643, which merges two field reports with one root cause:

• knowledge#312 — page-specific manual TOC rail
• knowledge#345 — full-bleed AI chat panel docked at the page edge

Both were working around the same wall: the only handle on the content region is the children render prop, which renders inside the padded, scrolling, header-topped inset. So consumers reached for :has() + !important overrides on [data-slot="sidebar-inset"] / [data-slot="sidebar-wrapper"], plus a MutationObserver on [data-state] and a hidden-trigger .click() to track sidebar collapse.

API

<SidebarLayout
  body={
    <>
      <TocRail />
      <SidebarLayout.ContentContainer header={<SidebarLayout.DefaultHeader />}>
        <SidebarLayout.Outlet />
      </SidebarLayout.ContentContainer>
      <AssistantPanel />
    </>
  }
/>

• body replaces the region beside the sidebar. As a flex sibling it reflows on collapse for free. Supersedes header/children; warns in dev when combined.
• SidebarLayout.ContentContainer — the stock content column (inset padding, pinned header slot, scroll region), extracted. It now owns the scroll ref, so useAppShellScrollContainer() keeps working inside a custom body. The default path renders the same component — one implementation, not two.
• SidebarLayout.Outlet / SidebarLayout.Trigger — page outlet and collapse toggle, for composing custom bodies.
• useAppShellSidebar() — { open, isMobile, setOpen, toggle }, inert outside a SidebarLayout. Replaces the MutationObserver + hidden-.click() workaround.

SidebarInset width change

w-[calc(100%-var(--sidebar-width))] → min-w-0.

That calc hardcoded "exactly one sibling, exactly --sidebar-width wide" — the assumption body breaks. It dates to the initial commit's scaffold (carried through the file split untouched, no comment, no test) and was really a workaround for a missing min-w-0: SidebarInset is w-full flex-1 with no min-w-0, so its flex min-width: auto resolved against w-full and overflowed the row by the sidebar's width. min-w-0 is the flex idiom already used five times elsewhere in that file.

Verified on /dashboard/products at 1280px: min-width: 0px, inset exactly 1024 = 1280 − 256, zero overflow on document, wrapper, or row.

Verified

• Four columns render with no overflow; assistant panel flush at the viewport edge; content column keeps its breadcrumb header and md:px-8 padding
• Sidebar collapse reflows every column; rail label flips via useAppShellSidebar()
• useAppShellScrollContainer() resolves inside a custom body
• Dark mode, mobile, and the Next.js example (default path, no body) all unchanged
• 1430 tests pass, no type errors, lint clean

Still to come

Pilot scope — docs, changeset, and tests for the new surface are not in this PR yet.

Open design question: body is configured at app level, but both reported use cases are page-level. The vite example drives it off useLocation(), which works but means shell config has to know page routes. Options written up in decisions/page-level-body-columns.md.