applySourceLineLayer

Inserts a CodeSourceLine compound between each com.stigway.metamodels.highway.CodeFunction box and its child instructions, grouping them by (repo, path, line):

Function → SourceLine → Instruction

Unlike the file layer this needs no representative pick — every instruction has exactly one source line, so the grouping is an exact partition. Instructions with no source line stay directly under the function. Boxes are scoped per function (line 42 of f is a distinct box from line 42 of g).

Must run after the function→instruction parenting is in place (loader) and before Enrichment.applyCompoundAsilMax (so the new boxes inherit the MAX-of-children ASIL) and icon assignment. No-op when enabled is false or no instruction carries a source line.