The board needs to resolve one leftward slide and merge turn. Implement the turn so a tile created by a merge cannot merge again during that same turn.
Build the implementation in JavaScript under /app, with /app/run.sh as its entry point. Read one JSON request from standard input and write one JSON result to standard output. Input is a list of rows of nonnegative integers where positive values are powers of two. For each row remove zeros, merge equal adjacent values once per original tile, and pad with zeros to the original width. Return board and score as the sum of newly merged values. Reject invalid tile values; rows may have different widths.
Reject inputs that violate an explicit validation rule with exit status 2, no standard output, and {"error":"invalid_input"} on standard error. For valid input, exit with status 0. Keep the implementation reusable for new inputs; it must work offline and must not depend on the example data being the only data it receives.
Repair or replace the unfinished /app/run.sh and add source files under /app. Malformed JSON must use the same invalid-input response. Do not read /tests or /oracle from the application.