Use per-page annotation spacing with inferred margin fallback

docs/final_output.md

@@ -10,13 +10,18 @@ extension **`.jpg`**, not `.jpeg`.
 
 ```python
 ANNOTATION_MARGIN_LEFT = 300
-ANNOTATION_LOW_MARGIN_LEFT = 400
+ANNOTATION_MARGIN_POINTS = [(10, 450), (20, 400), (35, 300)]
 ```
 
-These are added pixels at the 200 dpi annotation rendering scale. An answer
-uses the larger margin if any of its source fragments has a measured left
-margin below 20 mm (half the standard 40 mm). Unknown or insufficiently written
-pages retain the normal spacing. The 600 px comment width is unchanged, so
+The reference points are (measured margin in mm, added pixels at 200 dpi).
+Each page fragment gets its own spacing: 450 px at or below 10 mm, 400 px at
+20 mm, and 300 px at or above 35 mm, with linear interpolation between points.
+Unknown pages retain their unknown status but use the arithmetic mean of the
+copy's measured page margins, including during label-strip positioning.
+Inferred values never contribute to that mean. If no page has a measurement,
+`ANNOTATION_MARGIN_LEFT` supplies the default 300 px.
+The old `ANNOTATION_LOW_MARGIN_LEFT` setting is replaced by the reference points.
+The 600 px comment width is unchanged, so
 comments can still overlap handwriting on pages with very little whitespace.
 
 Automatic `crop-labels` saves per-page measurements in the Cutleft schema.
@@ -33,7 +38,10 @@ save their rendering geometry and reuse it when read back, even after settings
 change. Older batches without saved geometry keep their original 300 px offset.
 Do not overwrite a batch that still contains handwritten corrections to import.
 
-All fragments of one answer share one left offset. BGnot pads narrower answer
+Each fragment is positioned independently, with its annotation rectangles shifted
+by the same amount. Rectangles crossing page boundaries are split at those boundaries.
+Narrow pages receive any additional width on the right, so left padding does not
+stack with the configured allowance. BGnot pads narrower answer
 images with white without resizing handwriting; fitting the wider document to
 screen or paper may reduce the displayed scale, and return-PDF page breaks can
 change. The extra space is also used by Anot and Bnot.