Source-level sugar for Stock.Override: a parsedResultAction that lowers the lowercase, no-backtick surface

Override [ x via Sum, Coord at 0 via Sum ] T

into the honest marker form the type-checker plugin reads

Override [ "x" := Sum, At Coord 0 := Sum ] T

keeping a single infix operator (:=); at becomes the prefix marker At, and a bare lowercase selector becomes a Symbol literal. The rewrite is scoped to Override applications, runs before renaming, and reuses the original sub-trees (so spans survive); via/at elsewhere are untouched. Enabled by the same -fplugin Stock as the solver.