v0.58.0 — multi-AOI legacy iScan BIF stitching (closes #67)

Multi-AOI legacy iScan BIF stitching — place every scanned Area of Interest at
its own slide anchor (closes #67).

Fixed

• Legacy iScan BIF multi-AOI placement (#67). A legacy iScan slide may carry
  several Areas of Interest (AOIs) — separate scanned tissue regions, each a
  sub-grid of the global tile grid placed at its own origin (OS-2 has three
  AoiOrigin nodes, one unscanned). buildLegacyLayout previously stitched the
  whole frame grid as a single AOI, which overlaid the disjoint regions and left
  a visible seam through the large AOI on zoom. Following openslide's Ventana
  reader, the layout now pairs ImageInfo[i] with AoiOrigin[i] by document
  order, skips AOIScanned=false AOIs, and places each scanned AOI's local
  NumCols×NumRows grid at its own (Pos-X, Pos-Y) anchor (Pos-Y is measured
  from the AOI bottom, so it is Y-flipped into image space). Within each AOI the
  existing separable per-gap-average overlap model (#63) is reused. Single-AOI
  slides (OS-1: one AOI at origin 0) are the degenerate case and remain
  byte-identical. OS-2 L0 now reports the union hull across scanned AOIs
  (114951×76389) with the inter-AOI seam removed; reduced levels floor-halve.

Notes

• Per-tile raw/decoded bytes and Grid are unchanged (raw frame addressing).
  Only the stitched placement (Level.Size, ReadRegion/ReadRegionScaled/
  ScaledStrips/StitchedTile output) changes, and only for multi-AOI legacy
  slides. The other 10 formats and DP-generation BIF are untouched.
• OS-2.bif is a PHI/local-only fixture (gitignored); its committed
  OS-2.bif.json snapshot and the TestBIFGeometry/TestSlideParity pins are
  SHA/geometry-only and skip cleanly in CI when the slide is absent.