Padding tokens must not influence the pooled representation. Implement masked pooling so the denominator counts the included tokens and the all-masked case is handled explicitly.
Build the implementation in C++ 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 has a nonempty rectangular matrix of finite numeric vectors with positive width and a mask list of booleans, one per row. Return the column means of rows whose mask is true, rounded to six decimals. If all rows are masked return width zeros. Reject ragged matrices, nonboolean masks, mismatched lengths or nonnumeric/nonfinite vector entries.
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.