Fix sparse LongTable assay indexing

[mtran-code]

Updates LongTable() construction so assay indexing is built from observed assay rows instead of allocating a dense rowData x colData Cartesian product. It also preserves assay-specific keys, such as per-dose indices, so repeated observations for the same row/column pair remain recoverable through LongTable accessors.

Why

The previous constructor eagerly expanded every row/column combination before joining assays. For sparse treatment-response datasets, this can create extremely large temporary indices even when only a small fraction of row/column pairs have assay data.

The constructor also reduced assayIDs to only row and column identifiers, which dropped assay-specific identifiers needed to distinguish repeated measurements.

Changes

• Build assayIndex from observed assay records.
• Preserve assay-specific keys after row and column IDs in assayKeys.
• Keep assay-specific key columns in stored assay tables.
• Avoid duplicated key/value columns in assayCols().
• Add tests covering:
  • sparse assay index construction
  • assay-specific key preservation
  • summary assay expansion over observed keys

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