v0.60.1 — fix BIF Level.OverlapMode (OverlapStitched on stitched levels)

Fixed

• BIF Level.OverlapMode was OverlapNone on stitched levels. v0.60.0 added
  the OverlapMode field and wired DZI/SZI to set it, but the BIF reader was not
  updated — it set only the (correct) Overlapping bool, leaving OverlapMode at
  its zero value. So a stitched BIF level reported Overlapping=true but
  OverlapMode=OverlapNone, violating the documented invariant
  Overlapping == (OverlapMode != OverlapNone) and mis-reporting BIF as
  non-overlapping via the new enum. BIF now derives both fields from one source:
  overlapping levels report OverlapStitched, non-overlapping report
  OverlapNone. The Overlapping bool value is unchanged (it was already
  correct), so consumers gating on !Overlapping are unaffected; only the
  OverlapMode enum value is corrected. New TestOverlapModeInvariant
  (cross-format) and a BIF-specific OverlapStitched assertion guard it.