Fix OCR text layer misalignment with non-zero mediabox origins

Fixes #1630 where --redo-ocr would shift OCR text vertically on PDFs
with non-zero mediabox origins (e.g., [0, 100, width, height+100]).

The bug occurred in _graft_fpdf2_text_layer where the Form XObject BBox
was set to the text layer's mediabox [0, 0, w, h] instead of the base
page's mediabox [0, 100, w, h+100]. This caused a coordinate mismatch
between the BBox and the transformation matrix, resulting in text being
positioned incorrectly.

The fix changes line 450 in _graft.py to use base_mediabox instead of
mediabox, making the fpdf2 renderer consistent with the sandwich renderer
which already used base_mediabox correctly.

This issue commonly affected:
- JSTOR PDFs (generated by iText with cropping)
- Cropped PDFs from various tools
- PDFs with non-standard coordinate systems

Added regression test that creates a PDF with offset mediabox origin
and verifies --redo-ocr preserves coordinates correctly.

src/ocrmypdf/_graft.py

@@ -447,7 +447,7 @@ class OcrGrafter:
         xobj.Type = Name.XObject
         xobj.Subtype = Name.Form
         xobj.FormType = 1
-        xobj.BBox = mediabox
+        xobj.BBox = base_mediabox
 
         # Copy resources from text page's Resources to xobj
         # We need to handle this carefully since text_page is from a foreign PDF