v5.0.0

Adds a sampled request log and an offline replay harness. With MODELGATE_REQUEST_LOG set, an exact deterministic share of accepted requests is written as JSON lines containing only the six validated numeric inputs, the serving version, and the answer; nothing from the HTTP layer reaches the file. modelgate eval replays such a log through the same validation, encoding, and padded forward pass the service uses and reports MAE, p95 error, bias, a calibration table with expected calibration error per version, the divergence between two versions, and how many logged answers the replay reproduced (0 mismatches on the committed 300-record fixture, where v1 scores MAE 2.94 and v2 2.01). The README now carries a Releases table summarizing 1.0.0 through 5.0.0. 104 tests.