v0.59.0 — cross-axis drift in legacy iScan BIF stitching (closes #68)

Cross-axis drift in legacy iScan BIF stitching — model the per-column vertical
(and per-row horizontal) scanner-stage skew the joints record (closes #68).

Fixed

• Legacy iScan BIF per-column/per-row drift (#68). Adjacent camera frames
  are captured at a small cross-axis offset — horizontally-adjacent frames at
  a slight vertical shift, vertically-adjacent frames at a slight horizontal
  shift — a faint scanner-stage skew the scanner records as the <TileJointInfo>
  join's cross-axis overlap. The previous separable layout placed every tile
  in a grid row at the same Y (and every tile in a column at the same X),
  discarding that component; it accumulated into a visible per-column vertical
  shear on zoom (the "slightly wonky tile placement" symptom) and a ~0.05%
  height residual. buildLegacyLayout now integrates the full 2-D join
  displacement vectors — (tw−OverlapX, −OverlapY) horizontally,
  (−OverlapX, th−OverlapY) vertically — which separates into in-axis
  X[col]/Y[row] plus cross-axis yCol[col]/xRow[row] baselines, placing
  tile (c,r) at (X[c]+xRow[r], Y[r]+yCol[c]). The cross-axis sign and
  magnitude are pixel-validated against OS-2 cross-correlation and locked by a
  new seam-MAD gate (the modeled offset lowers the overlap-band MAD on all four
  legacy fixtures — AC1.592 21.2→6.6, 1_19 15.8→6.8). Clean-room: derived from
  the file's own joint overlaps, not a translation of openslide/bio-formats.

Notes

• Honoring the skew makes the stitched grid a faint parallelogram, so a legacy
  L0's reported Level.Size (and the floor-halved reduced levels) is slightly
  larger than openslide's nominal de-sheared extent by the integrated drift
  span (OS-1 +123×+174 px on a 105936×94125 hull; OS-2 115060×76560). openslide
  is the lower bound. Per-tile raw/decoded bytes and Grid are unchanged.
• DP-generation BIF (which uses the <Frame>-node layout, not
  buildLegacyLayout) and the other 10 formats are untouched.
• All legacy fixtures (OS-1/OS-2/AC1.592/1_19/S12-18199-1A) are PHI/local-only;
  the cross-axis pixel gate and dims/geometry pins skip cleanly in CI.