docs(plan): Task 5 must preserve blank lines in strip_page_artifacts

docs/superpowers/plans/2026-06-10-astroparse.md

@@ -346,6 +346,29 @@ curl -L -o backend/tests/fixtures/mowla_iyer_2024.pdf https://arxiv.org/pdf/2402
 ```
 Verify it's a PDF: `file backend/tests/fixtures/mowla_iyer_2024.pdf` → "PDF document".
 
+- [ ] **Step 1b: Fix `strip_page_artifacts` to preserve blank lines** (integration fix found in Task 4 review: `parse_pdf` rejoins lines and segments on blank lines, so artifact-stripping must NOT drop them). TDD: add to `backend/tests/test_parse_clean.py`:
+
+```python
+def test_strip_page_artifacts_preserves_blank_lines():
+    lines = ["First para.", "", "12", "Second para."]
+    assert strip_page_artifacts(lines) == ["First para.", "", "Second para."]
+```
+
+Run (FAIL), then change the filter in `strip_page_artifacts` so blank lines pass through (drop only artifacts):
+
+```python
+def strip_page_artifacts(lines: list[str]) -> list[str]:
+    return [
+        l for l in lines
+        if not l.strip()
+        or (not _PAGE_NUM.match(l.strip())
+            and not _ARXIV_STAMP.search(l)
+            and not _DOI_STAMP.match(l.strip()))
+    ]
+```
+
+Run (PASS — the existing 3 tests must also still pass; their inputs contain no blank lines).
+
 - [ ] **Step 2: Write failing tests**
 
 ```python