fix(host): race directory reads against the caller's signal; report aborts as cancelled

Every filesystem await in the browse scan (opendir and each read) now
races the signal through raceAbort, so a stalled network open/read stops
with a departed caller and an already-aborted request rejects even for
an empty level; the abandoned settlement is swallowed and an abandoned
open that still mints a handle is closed, never leaked. apiproxy maps an
aborted listing to the cancelled wire code, matching pickDirectory and
command.execute, instead of reporting a false internal failure. The
fixture spec call sites gain the wire signal argument the previous
commit's static lane flagged.

packages/client/connection/tests/fixture.spec.ts

@@ -320,13 +320,13 @@ describe('createFixtureApi', () => {
     const created = await api.host.createDirectory(req({ path: '/', name: 'srv' }))
     if (!created.result.ok) throw new Error('create failed')
     expect(created.result.value.path).toBe('/srv')
-    const listed = await api.host.listDirectory(req({ path: '/srv' }))
+    const listed = await api.host.listDirectory(req({ path: '/srv' }), new AbortController().signal)
     if (!listed.result.ok) throw new Error('list failed')
     expect(listed.result.value.crumbs).toEqual([
       { name: '/', path: '/', hidden: false },
       { name: 'srv', path: '/srv', hidden: false },
     ])
-    const root = await api.host.listDirectory(req({ path: '/' }))
+    const root = await api.host.listDirectory(req({ path: '/' }), new AbortController().signal)
     if (!root.result.ok) throw new Error('root list failed')
     expect(root.result.value.entries).toContainEqual({ name: 'srv', path: '/srv', hidden: false })
   })