Agent Observer 0.5.0a16

Compatible contract-validation fast path for ASCII strings; non-ASCII validation and frozen Mesh semantics are preserved. Source: 301b57a. Clean exported wheel SHA-256: 94705baebfa51a9664e9dbfa7603dcb932d6301146cf9a59017017c5285b3b17. Source gate: 448 tests with optional Mesh/jsonschema inputs enabled and no skips. Mesh reader and bridge selection use independent environments; the ordinary collector and writer remain independently selected.