v0.59.1 — precise subtile crop origin (fixes deep-zoom cross-level drift on legacy BIF)

Precise subtile crop origin — fixes deep-zoom cross-level drift on legacy
iScan BIF's non-square tiles (#68 follow-up).

Fixed

• Deep-level cross-level registration on legacy BIF (non-square tiles). The
  reduced-level subtile model (v0.56.0) computed a subtile's crop origin within
  its stored tile as q·(Dim>>i), which floors the per-subtile size. A stored
  tile packs 2ⁱ subtiles per axis across its full TileW/TileH, so flooring
  accumulates a crop-origin error of up to (2ⁱ−1)·frac px when the dimension
  is not a multiple of 2ⁱ. Legacy iScan tiles are non-square 1024×1360:
  1024 is a power of two (exact), but 1360/32 = 42.5 first loses precision
  at L5, drifting the vertical crop up to ~15 px at the bottom of each
  stored tile — a content shift that appeared as level-to-level "drift" when
  zooming out past L4. The crop origin now uses the precise round(q·Dim/2ⁱ).
  Cross-level registration measures ≤ 1 px at every level (was +6…+15 px at
  L5–L6); X was always clean (power-of-two width). DP-generation BIF (square
  1024×1024 tiles) and the other 10 formats are byte-identical.

Notes

• No API, Level.Size, placement, Grid, or tile-byte changes — only the
  pixel content sampled at deep reduced levels of non-square-tile legacy BIF.
  No fixture/geometry-pin changes. New TestReducedDeepLevelRegistration
  (local-only) + TestSubtileSourcePreciseCrop (CI-safe synthetic) gate it.