Unified sx<->compiler binding that subsumes the metatype declare/define
primitives AND the #compiler struct attribute. A named 'compiler' library
exposes the compiler's real types (layout-welded via extern(.zig), offsets
queried from the Zig type at compiler-build time + a build-time equality
assertion) and functions (comptime-only, host-call bridged). declare/
define/type_info become sx library code over register_*/find_type; the
projected meta.sx TypeInfo + hand marshaling are deleted; BuildOptions
migrates onto it and #compiler is removed. Includes the safety boundary
(curated export list, guarded mutators, comptime-only), the honest limit
(the ordering law stays, but stops leaking as 'weird stages' — dissolving
the 0141 class), a phased suite-green build order, and the open risks
(union(enum) welding, optional fields, LLVM offset emission).
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