refactor(web): hand session exports to browser downloads

The export endpoint already streams a ZIP response, but the web client immediately converted that response into a Blob. That forced the complete archive through JavaScript memory before a download could start and coupled transport, buffering, object-URL lifetime, and filename handling to the trajectory view.

Navigate a temporary download anchor directly to the export endpoint instead. The browser now owns streaming and HTTP failure presentation, while a standalone delivery module owns URL construction and filename sanitization. Focused tests cover the handoff, rejection behavior, and the assembled session view; the package README and feature note record the new ownership boundary.

packages/client/ui-trajectory/tests/export-log.spec.ts

@@ -1,12 +1,15 @@
-// @vitest-environment node
+// @vitest-environment jsdom
 /**
- * Session-log export filename derivation. The archive itself is produced and
- * streamed by the host (GET /api/session.export); this package only derives
- * the download filename and triggers the browser save.
+ * Session-log export browser delivery: safe filename derivation and a native
+ * download handoff that leaves the streamed response outside JavaScript.
  */
 
-import { describe, expect, it } from 'vitest'
-import { sessionLogZipFilename } from '../src/client/export-log.ts'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { downloadSessionLog, sessionLogZipFilename } from '../src/client/export-log.ts'
+
+afterEach(() => {
+  vi.restoreAllMocks()
+})
 
 describe('sessionLogZipFilename', () => {
   it('keeps safe session ids verbatim', () => {
@@ -22,3 +25,27 @@ describe('sessionLogZipFilename', () => {
     expect(sessionLogZipFilename('..')).toBe('dsh-session-__.zip')
   })
 })
+
+describe('downloadSessionLog', () => {
+  it('hands the descendant-inclusive endpoint directly to the browser', async () => {
+    const click = vi.spyOn(HTMLAnchorElement.prototype, 'click').mockImplementation(() => {})
+
+    await downloadSessionLog('session/with spaces')
+
+    expect(click).toHaveBeenCalledOnce()
+    const anchor = click.mock.contexts[0] as HTMLAnchorElement
+    const url = new URL(anchor.href)
+    expect(url.pathname).toBe('/api/session.export')
+    expect(url.searchParams.get('sessionId')).toBe('session/with spaces')
+    expect(url.searchParams.get('includeDescendants')).toBe('true')
+    expect(anchor.download).toBe('dsh-session-session_with_spaces.zip')
+  })
+
+  it('rejects when the browser download handoff fails', async () => {
+    vi.spyOn(HTMLAnchorElement.prototype, 'click').mockImplementation(() => {
+      throw new Error('download denied')
+    })
+
+    await expect(downloadSessionLog('session-root')).rejects.toThrow('download denied')
+  })
+})

packages/client/ui-trajectory/tests/views.spec.tsx

@@ -1141,39 +1141,26 @@ describe('timeline projection', () => {
 describe('session log export', () => {
   afterEach(() => {
     vi.unstubAllGlobals()
-    Reflect.deleteProperty(URL, 'createObjectURL')
     Reflect.deleteProperty(HTMLAnchorElement.prototype, 'click')
   })
 
   it('downloads the host-streamed ZIP with descendants on click', async () => {
-    // exportLog always fetches a URL instance, so the mock's shape stays narrow.
-    const fetchMock = vi.fn(async (input: URL) => {
-      expect(input.pathname).toBe('/api/session.export')
-      expect(input.searchParams.get('sessionId')).toBe(SID)
-      expect(input.searchParams.get('includeDescendants')).toBe('true')
-      return new Response('zip-bytes')
-    })
-    vi.stubGlobal('fetch', fetchMock)
-    const createObjectURL = vi.fn(() => 'blob:export')
-    URL.createObjectURL = createObjectURL
     const clickAnchor = vi.fn()
     HTMLAnchorElement.prototype.click = clickAnchor
     const b = await bench(historySnapshot(NODES))
     mount(b.slots)
     fireEvent.click(screen.getByRole('tab', { name: 'Trajectory' }))
     fireEvent.click(screen.getByRole('button', { name: 'Export session log' }))
-    await vi.waitFor(() => {
-      expect(fetchMock).toHaveBeenCalledOnce()
-    })
-    // The blob download lands a few microtasks after the fetch settles.
-    await vi.waitFor(() => {
-      expect(createObjectURL).toHaveBeenCalled()
-    })
-    expect(clickAnchor).toHaveBeenCalled()
+    await vi.waitFor(() => { expect(clickAnchor).toHaveBeenCalledOnce() })
+    const anchor = clickAnchor.mock.contexts[0] as HTMLAnchorElement
+    const url = new URL(anchor.href)
+    expect(url.pathname).toBe('/api/session.export')
+    expect(url.searchParams.get('sessionId')).toBe(SID)
+    expect(url.searchParams.get('includeDescendants')).toBe('true')
   })
 
-  it('surfaces the download failure in the visible alert bar', async () => {
-    vi.stubGlobal('fetch', vi.fn(async () => new Response('boom', { status: 404 })))
+  it('surfaces a browser handoff failure in the visible alert bar', async () => {
+    HTMLAnchorElement.prototype.click = vi.fn(() => { throw new Error('download denied') })
     const b = await bench(historySnapshot(NODES))
     mount(b.slots)
     fireEvent.click(screen.getByRole('tab', { name: 'Trajectory' }))
@@ -1181,7 +1168,7 @@ describe('session log export', () => {
     await vi.waitFor(() => {
       const alert = screen.queryByRole('alert')
       expect(alert).not.toBeNull()
-      expect(alert!.textContent).toContain('HTTP 404')
+      expect(alert!.textContent).toContain('download denied')
     })
   })
 })