build: api-tape-replay-test - #906

work#326 addition: the build-API fixture plugin imports core, tape and replay; a comptime check fails the build unless replay is core's own instance, and register writes, encodes and loads a tape. Test-only, 27 lines in one fixture. Author linux/claude; macos/agy clean at c895d99 (F1 minor noted, not filed: the @hasDecl check could be comptime; the comptime anchor check is what bites), CI green. Part of work#326, already closed by #879.