Pure runtime/str.c addition: str_alloc helper + ailang_int_to_str +
ailang_float_to_str. No codegen, checker, or build-pipeline changes
in this iter — the IR-side wiring (declare lines, lowering, checker
install, rc.c always-linked) all batch into hs.4. Acceptance gate:
runtime continues to build and link with the new symbols present but
unreferenced (clang -O2 dead-strips them until hs.4 wires a caller).
Two tasks: (1) append three functions + includes + extern decl to
str.c, verify via clang -c + nm; (2) full workspace + cross_lang +
compile_check + check sweep stays green.
