IFE resolution: read the Iris scale-relative convention, no vendor override (#81)

IFE Magnification/MPP read the Iris scale-relative convention — verified against the
Iris-Codec encoder source, with no per-vendor override (#81 follow-up).

Fixed

• IFE Metadata.Magnification / Metadata.MPP read the Iris resolution convention.
  v0.54.0 mis-framed the root cause as "an encoder stamped a downsampled level's value
  into the header" and special-cased aperio.AppMag / aperio.MPP. The actual Iris
  convention — confirmed against the Iris-Codec encoder source
  (Iris-Codec/src/IrisCodecEncoder.cpp, READ_OPENSLIDE_METADATA, which writes
  micronsPerPixel = MPP_finest × max_scale and magnification = objective / max_scale,
  front().downsample == max_scale) — is that the METADATA header stores
  scale-relative quantities: magnification is a coefficient and micronsPerPixel
  is anchored at the lowest-resolution layer. opentile-go inverts that:
  Magnification = magnification × max_scale and MPP = micronsPerPixel / max_scale
  (max_scale = the finest layer's scale). The raw header values stay in
  MagnificationFromHeader / MPPFromHeader.

Removed

• The aperio.* override is gone. v0.54.0/first-cut-v0.54.1 special-cased the
  source scanner's aperio.AppMag / aperio.MPP (or ImageDescription banner) to
  "correct" a header that disagreed with the convention. That existed solely to prop up
  one fixture (cervix_2x_jpeg.iris) whose header is stale — computed for a 4-layer
  max_scale = 64 pyramid but the file ships a 9-layer max_scale = 256 ladder, so the
  convention at its real max_scale = 256 would give an impossible 160×. The
  convention is the spec; a disagreeing header is a bug in the file, so the override
  (aperioL0Resolution + helpers) was removed and the fixture's header is corrected at
  the source instead (see Added). aperio.* values remain available via
  Properties["iris.aperio.*"].

Added

• cmd/ifefixheader — a small utility that rewrites an .iris METADATA header's
  scale-relative resolution fields to be conformant with the file's own pyramid. It
  reads max_scale from LAYER_EXTENTS and writes
  magnification = appmag / max_scale, micronsPerPixel = mpp × max_scale from the
  supplied true L0 objective / MPP. Used to fix the public cervix_2x_jpeg.iris
  fixture's stale header: go run ./cmd/ifefixheader -appmag 40 -mpp 0.262968 ….

Documentation

• docs/formats/ife.md — documents the reversed Iris layer numbering, the
  scale-relative header semantics, the × / ÷ max_scale derivation (cited to the
  encoder source), the no-override stance + cmd/ifefixheader workflow, and corrects an
  earlier "cervix is a 2× downsample of a 253,952 × 177,152 original" note (the embedded
  Aperio metadata reports native 126940×88416 @ 0.262968 = the finest layer). The
  public 425248_JPEG/AVIF.iris references carry no resolution metadata at all. The
  local sample_files/ife/ife-format-spec-for-opentile-go.md reference gains a matching
  note.