encode_json(put d k v): null on the interpreter, stack underflow on bytecode/HFIR; narrow the difftest HFIR skip #161

From Motoko's #160 review. This is also on base.

1. (encode_json (put d k v)) prints null on the interpreter but stack-underflows on -run-bc and HFIR. put should return the updated dict on all three paths, with parity tests.
2. The difftest HFIR skip matches the generic 'not in the Phase-1 executable subset' error, so it would hide future unsupported constructs, and skipped fixtures count as Passed. Narrow it to an explicit list (optimize_signature) and print a SKIP line.