Add regression tests for existing behaviour - #608

Several behaviours that users rely on had no pinning test, so plausible regressions passed silently: interpolation immediately after multibyte text, the anchor that keeps an escaped nested reference literal next to a bare dollar sign, indented comments, and UTF-16 and UTF-32 files carrying a byte order mark with a matching file encoding. Two test helpers also ignored their expected-name argument. The immutable load test asserted only that getenv was unchanged, which the safe factories never write anyway, so it passed vacuously; it now asserts the load return value and each environment channel. This adds targeted coverage and fixes the helpers. This is stacked on #607.