convert --to ome-tiff drops magnification (and make/software/datetime)

[cornish]

Summary

convert --to ome-tiff drops several source metadata fields that the SVS→svs and
SVS→tiff paths (partly) preserve. On a 20× Aperio source:

field	source	ome-tiff output
mpp	0.499	0.499 ✅
magnification	20	0 (dropped)
make	Aperio	(empty)
software	Aperio Image Library v11.2.1	(empty)
datetime	2009-12-29T09:59:15Z	(empty)

MPP survives; magnification and the identity fields do not.

$ wsitools convert --to ome-tiff -f -o out.ome.tiff CMU-1-Small-Region.svs
$ wsitools info out.ome.tiff | grep -iE 'magnif|make'   # no Magnification / Make line

Impact

OME-TIFF outputs silently lose objective magnification (and make/software/datetime).
Magnification is load-bearing for downstream analysis. Found by the automated CLI
matrix audit (scripts/qa/audit.sh, cross-container metadata check).

Fix direction

Populate the OME-XML with the source magnification (Objective NominalMagnification)
and the make/software/acquisition-date fields in the OME-TIFF writer path
(cmd/wsitools/convert_tiff.go OME description synthesis / SyntheticOMEDescription).

[cornish]

Magnification is fixed on main (1f1c249): both the ome-tiff tile-copy and re-encode paths now emit the <Instrument>/<Objective NominalMagnification> block via SyntheticOMEDescriptionWithMag, so a 20x source reads back 20x (regression test TestConvertOMETIFFPreservesMagnification).

Remaining: the identity fields (make / software / datetime) are still not carried in the OME-XML — that needs <Instrument><Microscope Manufacturer=…> / <Image AcquisitionDate> (and confirming opentile-go's OME reader surfaces them). Leaving this open for that part.

[cornish]

Identity fields addressed on main (0044cab):

• AcquisitionDate — now emitted as <Image><AcquisitionDate> and round-trips through opentile (SVS …09:59:15Z → ome-tiff reads back 09:59:15Z). ✅
• Make/Model/Serial — now emitted as a standard OME <Instrument><Microscope Manufacturer/Model/SerialNumber> block (+ <InstrumentRef>), read by Bio-Formats/QuPath. Verified: SVS → ome-tiff emits <Microscope Manufacturer="Aperio" SerialNumber="CPAPERIOCS"/>. ✅ write side
• Software — carried via the OME Creator attribute (wsitools/… (from <src software>)), promoted by opentile to the typed Writer field.

Regression test: TestConvertOMETIFFIdentityFields.

Remaining (upstream): opentile-go's OME reader doesn't yet surface make/model/serial from the typed <Microscope> element, so wsitools info still shows them empty even though the OME-XML is correct — filed as WSILabs/opentile-go#106. Once that lands, make/model round-trip through opentile too. Closing the wsitools write-side here; the read-side is tracked upstream.