Three more value-observable behavior families ported from the C corpus
to in-language @test, routed by the ratified observability rule (value
-> test/lang @test; process-outcome stays in runww). Additive: the .c
sources keep running in $(TESTS), so no byte-id coverage is removed --
de-dup is deferred to fold 6 (task #12).
906_callret_unsigned_arith -> callret_unsigned_test: call-result
unsigned opcode select keyed by callee return type (#168, the N_CALL
twin of gunsigned's module-global #134); operands flow through real
calls so the return-type arm is exercised, not N_IDENT.
912_sar_shr -> sar_shr_test: signed >> / >>= must emit SAR not SHR
(#136); asserts the i64/i32 value directly, dropping the C 8-bit
exit-code encoding.
793_widen_pad_zero -> widen_pad_test: widening a narrow value into a
wider tagged slot zeroes the high pad words (#227).
