fix: add SidecarRoundtrip tests for D2-6

SPECGAPS.md

@@ -143,7 +143,7 @@ All reconciled to follow code. Specs must be self-consistent and match code.
 | ~~D2-3~~ | ~~CreateAndPublishScript rule~~ | ~~script.allium:160~~ | **Resolved:** `create_and_publish_script/1` implemented in scripts.ex, 4 tests added (happy path with file assertions + macro entrypoint default + invalid Lua rejection + slug dedup on title conflict) |
 | ~~D2-4~~ | ~~UniqueScriptSlug dedup~~ | ~~script.allium:115~~ | **Resolved:** test added asserting two scripts with same title produce unique slugs (`dup-slug` → `dup-slug-2`) |
 | D2-5 | ~~FrontmatterRoundtrip invariant~~ | post.allium:223 | **Resolved:** test added — creates post with all fields, publishes, parses file back via `Frontmatter.parse_document`, asserts every field (id, title, slug, excerpt, status, author, language, doNotTranslate, templateSlug, tags, categories, createdAt, updatedAt, publishedAt, body) matches the published DB record |
-| D2-6 | SidecarRoundtrip invariant | media.allium:198 | Write test: write sidecar, read back, assert all fields match |
+| ~~D2-6~~ | ~~SidecarRoundtrip invariant~~ | ~~media.allium:198~~ | **Resolved:** 2 tests added — full roundtrip (write sidecar, parse via `Sidecar.parse_document/1`, assert all fields match DB) + nil conditional fields absent from parsed sidecar |
 | D2-7 | ConditionalPostFields: nil fields absent from frontmatter | frontmatter.allium:398 | Write test: post with nil excerpt/author/language → fields not in file |
 | D2-8 | ConditionalMediaFields: nil fields absent from sidecar | frontmatter.allium:417 | Write test: media with nil title/alt → fields not in sidecar |
 | D2-9 | max_posts_per_page 1..500 constraint | metadata.allium:75-77 | Write test: values outside range rejected |

test/bds/media_test.exs

@@ -642,6 +642,81 @@ defmodule BDS.MediaTest do
     assert %{language: ["has already been taken"]} = errors_on(changeset)
   end
 
+  test "SidecarRoundtrip: writing then parsing a sidecar yields matching DB metadata", %{
+    project: project,
+    temp_dir: temp_dir
+  } do
+    source_path = Path.join(temp_dir, "roundtrip.txt")
+    File.write!(source_path, "hello media")
+
+    assert {:ok, media} =
+             BDS.Media.import_media(%{
+               project_id: project.id,
+               source_path: source_path,
+               title: "Roundtrip Title",
+               alt: "Roundtrip Alt",
+               caption: "Roundtrip Caption",
+               author: "Tester",
+               language: "en",
+               tags: ["alpha", "beta"]
+             })
+
+    sidecar_path = Path.join(temp_dir, media.sidecar_path)
+    {:ok, contents} = File.read(sidecar_path)
+    {:ok, parsed} = BDS.Sidecar.parse_document(contents)
+
+    assert parsed["title"] == media.title
+    assert parsed["alt"] == media.alt
+    assert parsed["caption"] == media.caption
+    assert parsed["tags"] == media.tags
+    assert parsed["id"] == media.id
+    assert parsed["originalName"] == media.original_name
+    assert parsed["mimeType"] == media.mime_type
+    assert parsed["size"] == media.size
+    assert parsed["author"] == media.author
+    assert parsed["language"] == media.language
+    assert parsed["createdAt"] == media.created_at
+    assert parsed["updatedAt"] == media.updated_at
+  end
+
+  test "SidecarRoundtrip: conditional fields are absent from parsed sidecar when nil", %{
+    project: project,
+    temp_dir: temp_dir
+  } do
+    source_path = Path.join(temp_dir, "minimal.txt")
+    File.write!(source_path, "hello media")
+
+    assert {:ok, media} =
+             BDS.Media.import_media(%{
+               project_id: project.id,
+               source_path: source_path
+             })
+
+    assert media.title == nil
+    assert media.alt == nil
+    assert media.caption == nil
+    assert media.author == nil
+    assert media.language == nil
+
+    sidecar_path = Path.join(temp_dir, media.sidecar_path)
+    {:ok, contents} = File.read(sidecar_path)
+    {:ok, parsed} = BDS.Sidecar.parse_document(contents)
+
+    refute Map.has_key?(parsed, "title")
+    refute Map.has_key?(parsed, "alt")
+    refute Map.has_key?(parsed, "caption")
+    refute Map.has_key?(parsed, "author")
+    refute Map.has_key?(parsed, "language")
+
+    assert Map.has_key?(parsed, "id")
+    assert Map.has_key?(parsed, "originalName")
+    assert Map.has_key?(parsed, "mimeType")
+    assert Map.has_key?(parsed, "size")
+    assert Map.has_key?(parsed, "tags")
+    assert Map.has_key?(parsed, "createdAt")
+    assert Map.has_key?(parsed, "updatedAt")
+  end
+
   defp errors_on(changeset) do
     Ecto.Changeset.traverse_errors(changeset, fn {message, opts} ->
       Regex.replace(~r"%{(\w+)}", message, fn _, key ->