feat(analysis): position-aware OCR seam dedup for sliced pages

Boundary text duplicated across slices (often with slightly different,
cropped transcriptions) survived the text-only merge. The OCR pass now
asks for a per-piece vertical position and the merge uses it:

- OcrResult gains optional `y` (fraction 0..1 of the slice); the Pass-A
  OCR schema/prompt request it (combined path leaves it None). Not
  persisted.
- merge_ocr takes each slice's working-image y-band and maps `y` to a
  page-global position. A seam pair is a duplicate when position-close
  (same kind) OR text-similar, so a mis-OCR'd boundary line is caught even
  when the text differs. The kept copy is the one more central in its slice
  (less cropped); falls back to keep-longer when positions are missing.
- Self-calibrates the model's y values (pixels vs. fraction) and ignores
  degenerate columns, so a bad localizer can't over-merge.

Tests: position pairs differing texts and keeps the less-cropped copy;
text-only fallback (dedup/keep-longer/non-adjacent/order) still holds.

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

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.78.0"
+version = "0.79.0"
 edition = "2021"
 default-run = "mangalord"