fix(host): keep the child pane reachable on narrow viewports; surface truncated levels

The Miller row now scrolls horizontally with the child pane pinned into
view when its preview lands, so descent stays reachable when the dialog
is narrower than two fixed panes. The dialog also says when a visible
level was cut at the backend's complete-result bound (817's maxEntries)
instead of letting the tail of a huge directory go silently missing.

packages/host/directory-picker-browse/tests/client-flow.spec.tsx

@@ -19,6 +19,7 @@ const homeListing: DirectoryListing = {
   home: HOME,
   crumbs: [{ name: '/', path: '/', hidden: false }, { name: 'u', path: HOME, hidden: false }],
   entries: [{ name: 'Documents', path: `${HOME}/Documents`, hidden: false }],
+  truncated: false,
 }
 
 async function bench() {

packages/host/directory-picker-browse/tests/directory-browser.spec.tsx

@@ -27,6 +27,7 @@ function listingFor(path?: string): DirectoryListing {
         { name: '.config', path: `${HOME}/.config`, hidden: true },
         { name: 'Documents', path: DOCS, hidden: false },
       ],
+      truncated: false,
     },
     [DOCS]: {
       path: DOCS,
@@ -38,6 +39,7 @@ function listingFor(path?: string): DirectoryListing {
         { name: 'Documents', path: DOCS, hidden: false },
       ],
       entries: [{ name: 'harness', path: HARNESS, hidden: false }],
+      truncated: false,
     },
     [HARNESS]: {
       path: HARNESS,
@@ -50,6 +52,7 @@ function listingFor(path?: string): DirectoryListing {
         { name: 'harness', path: HARNESS, hidden: false },
       ],
       entries: [],
+      truncated: false,
     },
   }
   const found = tree[target]
@@ -205,6 +208,7 @@ describe('DirectoryBrowser', () => {
         { name: 'data', path: '/srv/data', hidden: false },
       ],
       entries: [],
+      truncated: false,
     }
     mount({ listDirectory: vi.fn(async () => outside) })
     await waitFor(() => { expect(screen.getByRole('button', { name: 'data' })).toBeTruthy() })
@@ -253,6 +257,7 @@ describe('DirectoryBrowser', () => {
       path: `${HOME}/fresh`, home: HOME,
       crumbs: [...listingFor(HOME).crumbs, { name: 'fresh', path: `${HOME}/fresh`, hidden: false }],
       entries: [],
+      truncated: false,
     }
     b.listDirectory.mockImplementation((path?: string) =>
       new Promise<DirectoryListing>((settle) => {
@@ -519,6 +524,7 @@ describe('DirectoryBrowser', () => {
           path: `${DOCS}/fresh`, home: HOME,
           crumbs: [...listingFor(DOCS).crumbs, { name: 'fresh', path: `${DOCS}/fresh`, hidden: false }],
           entries: [],
+          truncated: false,
         }
       }
       if (path === DOCS) {
@@ -648,7 +654,7 @@ describe('DirectoryBrowser', () => {
   })
 
   it('names the create target by its path when the level reports no crumbs', async () => {
-    const bare: DirectoryListing = { path: '/srv/data', home: HOME, crumbs: [], entries: [] }
+    const bare: DirectoryListing = { path: '/srv/data', home: HOME, crumbs: [], entries: [], truncated: false }
     mount({ listDirectory: vi.fn(async () => bare) })
     await waitFor(() => { expect(screen.getByRole('button', { name: 'browser.newFolder' })).toBeTruthy() })
     await waitFor(() => {
@@ -676,6 +682,35 @@ describe('DirectoryBrowser', () => {
     await waitFor(() => { expect(screen.queryByLabelText('browser.folderName')).toBeNull() })
   })
 
+  it('says a level is incomplete when the backend cut it at its bound', async () => {
+    const cut = { ...listingFor(HOME), truncated: true }
+    mount({ listDirectory: vi.fn(async () => cut) })
+    await screen.findByText('browser.truncated')
+  })
+
+  it('flags a truncated child preview under a complete level', async () => {
+    mount({
+      listDirectory: vi.fn(async (path?: string) =>
+        (path === DOCS ? { ...listingFor(DOCS), truncated: true } : listingFor(path))),
+    })
+    await waitFor(() => { expect(screen.getByRole('listitem')).toBeTruthy() })
+    expect(screen.queryByText('browser.truncated')).toBeNull()
+    fireEvent.click(rowButton(screen.getByRole('listitem')))
+    await waitFor(() => { expect(columns()).toHaveLength(2) })
+    await screen.findByText('browser.truncated')
+  })
+
+  it('pins the child pane into view when its preview lands (narrow viewports scroll the miller row)', async () => {
+    mount()
+    await waitFor(() => { expect(screen.getByRole('listitem')).toBeTruthy() })
+    const row = document.querySelector('[class*=millerRow]') as HTMLElement
+    // jsdom does no layout: stub the overflow width the effect pins against.
+    Object.defineProperty(row, 'scrollWidth', { value: 640, configurable: true })
+    fireEvent.click(rowButton(screen.getByRole('listitem')))
+    await waitFor(() => { expect(columns()).toHaveLength(2) })
+    await waitFor(() => { expect(row.scrollLeft).toBe(640) })
+  })
+
   it('starts back at home on reopen', async () => {
     const b = mount()
     await waitFor(() => { expect(screen.getByRole('listitem')).toBeTruthy() })