refactor(web): open produced files through the Host, not over HTTP

Scope decision: previews for a browser that is not on the Host machine are
not supported. With that settled, host.openPath answers the supported case
completely — a file:// document in a real browser has full page capabilities
and no reach into /api — and the HTTP serving this branch had built answered
only the unsupported one.

Removed: the /f route and its listener, the workspace-file URL shape,
ApiProxy.workspaceRootOf, ConnectionHandle.fileUrl, and the port published
into the index page.

Kept, and finished:
- the produced-files row a turn ends with, derived from mutation locations;
- the path link now reads as a link at rest, not only on hover — the reported
  "I can't open what it made" was this, sitting on a working capability;
- the Host opener prefers the default BROWSER for .html/.htm/.xhtml/.svg, so
  a developer who binds .html to an editor still gets a rendered page
  (macOS via the LaunchServices https handler, Linux via $BROWSER, every
  failure falling back to the default application).

The retired designs and their measurements stay in the Agent Note, including
why same-origin serving was unsafe and why the sandbox that fixed it broke
the pages invisibly.

packages/client/runtime/tests/client-apply.spec.ts

@@ -26,7 +26,6 @@ async function mount(): Promise<Bench> {
   const bench: Bench = { ctx, api, sinks: undefined, stopped: 0 }
   const handle: ConnectionHandle = {
     api,
-    fileUrl: () => undefined,
     start: (sinks) => {
       bench.sinks = sinks
       return { stop: () => { bench.stopped += 1 } }

packages/client/runtime/tests/wire-events.spec.ts

@@ -20,7 +20,6 @@ async function mount(): Promise<Bench> {
   const bench: Bench = { ctx, sinks: undefined }
   const handle: ConnectionHandle = {
     api,
-    fileUrl: () => undefined,
     start: (sinks) => {
       bench.sinks = sinks
       return { stop: () => {} }