fix: compute exact pyramid resolutions to keep THUMBNAIL levels aligned (slim#318)

[igoroctaviano]

Summary

Fixes the annotation misplacement reported in ImagingDataCommons/slim#318, where the presence of a THUMBNAIL image causes the slide to "jump" and annotations to render in the wrong place while zooming.

Root cause

_computeImagePyramid computed each level's resolution (zoom factor) as baseTotalPixelMatrixColumns / totalPixelMatrixColumns and then rounded it to the nearest integer (Math.round).

• VOLUME levels are clean (power-of-two style) downsamples of the base level, so their ratio is already (essentially) an integer and rounding is harmless.
• THUMBNAIL images are not exact fractions of the base level. Confirmed against the repo's own TCGA-LUAD fixture:

flavor	columns	base/cols (true)	rounded (old)
THUMBNAIL	803	57.7036	58
VOLUME	2896	16.0000	16
VOLUME	11584	4.0000	4
VOLUME	46336	1.0000	1

Rounding 57.70 → 58 is a ~0.5% scale error (~238 px across the slide). It stretches the thumbnail level so its upsampled image content drifts toward the bottom-right relative to the (correctly placed) vector annotation layer. This is why disabling thumbnails (skipThumbnails) "fixed" it — the remaining volume-only pyramid has integer ratios.

This is distinct from, but compounds, the transient nearest-neighbor upsampling shift discussed in openlayers/openlayers#12768.

Fix

Use the exact column ratio for every level instead of rounding. Levels are processed base → top, so each zoom factor is naturally strictly greater than the previous; a small floating-point guard preserves OpenLayers' requirement that resolutions be unique and strictly descending.

Test plan

• New src/pyramid.test.js regression tests: thumbnail level is kept, every resolution equals the exact ratio, the thumbnail factor is not rounded to an integer, and resolutions remain unique/strictly descending.
• Full suite green: 162 passed.
• biome check clean.
• Production bundle builds (webpack-bundle).

Follow-up

After release, ImagingDataCommons/slim can drop the skipThumbnails: true workaround and re-enable thumbnails for faster initial load (see slim#318 discussion).