Fix empty sequence column options and add a loading state

[kevindetry-milaboratories]

Problem 1: empty "Sequence column to match" for bulk datasets

The dropdown is empty when the selected dataset is a bulk MiXCR dataset and the project also contains a single-cell MiXCR block. targetOptions returns [], the dropdown shows no entries, and Run stays disabled without a message.

Cause

model/src/index.ts, targetOptions output. The probe that decides whether a dataset is paired calls getAnchoredPColumns with a selector that has name and domain but no axes. A selector without axes matches the whole result pool. In a mixed project the probe hits the single-cell block's pl7.app/vdj/sequence columns with scClonotypeChain/index: "primary", so isPaired becomes true for the bulk dataset. The paired matcher then requires that domain on the bulk clonotypeKey axis. No column has it, so the result is empty.

The probe came in with 1.6.0 (#60). Before that, pairing was decided from the axis name only, which is correct for bulk MiXCR data.

Fix

Add axes: [{ anchor: "main", idx: 1 }] to the pairing probe and to the scFv probe. This is the same axis constraint the matchers below them already use. Axis 1 is the clonotype axis by construction: datasetOptions only accepts anchors with exactly two axes, pl7.app/sampleId at 0 and the clonotype, single-cell clonotype, or variant key at 1.

Problem 2: abrupt update after a dataset change

After a dataset change the dropdown kept the options of the old dataset until the new list arrived, then switched at once. A stale target could stay selected.

Fix

• Model: targetOptions returns { datasetRef, options }, the options tagged with the dataset they were computed for.
• UI: the dropdown gets undefined as options until the options for the selected dataset arrive. PlDropdown then shows a spinner and disables the field. When the list arrives, a target that is not in the list is cleared and the label is refreshed. Without a dataset the dropdown is disabled with the hint "Select a dataset first".

Test

Dev build loaded into a project with one 10x MiXCR block and one bulk MiXCR block on app.hz.platforma.bio:

Dataset	Published 1.7.0	This branch
Bulk IGHeavy/umi-count	0 options	16 options (FR1..FR4, CDR1..CDR3, VDJRegion, nt and aa)
10x abundance/IG/cell-count	32 options	32 options

Dataset switch 10x -> bulk with "Heavy CDR3 aa Primary" selected, sampled from the DOM: old value shown for 100 ms, then spinner and disabled field for 2.6 s, then the new options with the target cleared.

pnpm build:dev-no-software and pnpm check pass.