refactor(classifier): rename IMAGE_COVERAGE_SCANNED → IMAGE_HEAVY_THRESHOLD

Round-1 review nits. The constant now only gates the diagnostic `image_heavy`
flag (not routing), so the old name was misleading. Rename + reword its comment
to state the diagnostic-only intent. Also add a classify_pdf symmetry test
(`test_classify_pdf_image_heavy_clean_text_stays_fast`) pinning that a full-page
raster image with a clean text layer routes fast on the classify_pdf path too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_doc_classifier.py

@@ -118,6 +118,23 @@ def _image_with_mashed_text_pdf(pages: int = 2) -> bytes:
     return data
 
 
+def _image_with_clean_text_pdf(pages: int = 2) -> bytes:
+    # Full-page image with a CLEAN embedded text layer -- a scan carrying a good
+    # OCR layer, or a figure-heavy digital page. Image-heavy but usable text.
+    doc = pymupdf.open()
+    pix = pymupdf.Pixmap(pymupdf.csRGB, pymupdf.IRect(0, 0, 600, 850))
+    pix.clear_with(255)
+    img = pix.tobytes("png")
+    del pix  # Pixmap holds native memory; release it before the loop
+    for _ in range(pages):
+        page = doc.new_page(width=595, height=842)
+        page.insert_image(page.rect, stream=img)
+        page.insert_text((50, 60), "Hello world this is clean text. " * 8)
+    data: bytes = doc.tobytes()
+    doc.close()
+    return data
+
+
 def test_scanned_flag_when_no_text_layer():
     c = clf.classify_pdf(_full_page_image_pdf())
     assert c.total_chars == 0
@@ -250,6 +267,16 @@ def test_image_heavy_clean_text_stays_fast():
     assert all(p.needs_ocr is False for p in c.pages)
 
 
+def test_classify_pdf_image_heavy_clean_text_stays_fast():
+    # classify_pdf symmetry with test_image_heavy_clean_text_stays_fast: full-page
+    # raster images WITH a clean embedded text layer are image_heavy but route
+    # fast -- coverage is diagnostic, not routing, on the classify_pdf path too.
+    c = clf.classify_pdf(_image_with_clean_text_pdf())
+    assert c.recommended_tier == "fast"
+    assert "image_heavy" in c.flags
+    assert c.mean_text_quality >= clf.MIN_TEXT_QUALITY
+
+
 def test_scan_signal_ignored_when_coverage_low():
     full, bounds = _two_page(_CLEAN, _CLEAN)
     c = clf.classify_from_text(full, bounds, image_coverage=[0.1, 0.0])