fix(microprints): fix source viewer showing stale content when switching files

The source_viewer component used static DOM IDs (id='source-viewer')
with phx-update='ignore'. When switching expanded files, LiveView
reused the same DOM element but phx-update='ignore' prevented content
from being updated, showing the previous file's source.

- Override source_viewer/1 in MicroprintsLive with unique per-file IDs
  generated via :erlang.phash2(file_path)
- Add test verifying expand-switch shows correct source content
- Add test for highlight/expand coupling (collapse when highlighting
  a different file)
- All 160 tests pass

app/lib/firehose_web/live/microprints_live.ex

@@ -76,12 +76,11 @@ defmodule FirehoseWeb.MicroprintsLive do
                   highlighted_line={@highlighted_line}
                 />
 
-                <%= if @expanded_path == path and source do %>
-                  <.source_viewer
+                <%= if @expanded_path == path and source do %>                  <.source_viewer
                     content={source}
                     highlighted_line={@highlighted_line}
                     language="elixir"
-                    id={"source-viewer-" <> path}
+                    file_path={path}
                   />
                 <% end %>
               <% else %>
@@ -241,5 +240,23 @@ defmodule FirehoseWeb.MicroprintsLive do
   # Delegate to MicroprintComponent
   defdelegate microprint(assigns), to: MicroprintComponent
   defdelegate microprint_legend(assigns), to: MicroprintComponent
-  defdelegate source_viewer(assigns), to: MicroprintComponent
+
+  # Custom source_viewer with unique DOM IDs per file to prevent LiveView
+  # DOM patching bugs when switching expanded files.
+  def source_viewer(assigns) do
+    assigns = assign(assigns, :viewer_id, "source-viewer-" <> Integer.to_string(:erlang.phash2(assigns.file_path, 1_000_000)))
+
+    ~H"""
+    <div
+      id={@viewer_id}
+      class="source-viewer mt-2 max-h-96 overflow-auto rounded font-mono text-xs"
+      phx-hook="SourceViewer"
+      data-highlighted-line={@highlighted_line}
+      data-language={@language}
+      style="background: var(--sv-bg); color: var(--sv-text);"
+    >
+      <pre id={@viewer_id <> "-pre"} phx-update="ignore" class="m-0 p-2"><code class={"language-#{@language || "plaintext"}"}><%= @content %></code></pre>
+    </div>
+    """
+  end
 end