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/test/firehose_web/live/microprints_live_test.exs

@@ -93,11 +93,43 @@ defmodule FirehoseWeb.MicroprintsLiveTest do
       |> element("svg rect[phx-value-line=\"1\"][phx-value-path=\"#{file_b}\"]")
       |> render_click()
 
-      # BUG: file A should be collapsed when highlighting a different file
-      # Currently file A stays expanded while the highlight is on file B
+      # file A should be collapsed when highlighting a different file
       html = render(view)
       refute html =~ "Collapse",
              "file A should be collapsed after highlighting a different file, but the Collapse button is still visible (expanded_path is uncoupled from highlighted_path)"
     end
+
+    test "switching expand from file A to file B shows file B's source content, not file A's", %{conn: conn} do
+      files = MicroprintsLive.scan_source_files()
+      assert length(files) >= 2, "Need at least 2 files to test source switching"
+
+      [file_a, file_b | _] = files
+
+      {:ok, view, _html} = live(conn, ~p"/microprints")
+
+      # Expand file A
+      view
+      |> element("button[phx-value-path=\"#{file_a}\"]", "Expand")
+      |> render_click()
+
+      # Verify file A's source is shown (check for module def from microprints_live.ex)
+      html = render(view)
+      assert html =~ "FirehoseWeb.MicroprintsLive",
+             "When file A is expanded, its source should be visible"
+
+      # Expand file B (should auto-collapse A)
+      view
+      |> element("button[phx-value-path=\"#{file_b}\"]", "Expand")
+      |> render_click()
+
+      # Verify file B's source is shown, NOT file A's
+      html = render(view)
+      assert html =~ "Firehose.Application",
+             "When file B is expanded, its source should be visible (not file A's source)"
+
+      # File A's button should now say "Expand" (collapsed)
+      refute html =~ ~s(phx-value-path="#{file_a}".*Collapse),
+             "file A should be collapsed after switching expand to file B"
+    end
   end
 end