The sampler should return values on the piecewise linear signal defined by the supplied knots. Implement it so queries at knots and between knots follow the same interpolation contract.
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 at least two [x,y] knots with strictly increasing x and query positions. All coordinates must be finite numbers. Interpolate linearly between neighboring knots, including exact endpoints; return values rounded to six decimals in query order. Reject repeated/decreasing knot positions, outside queries or invalid numeric values.
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.