c_runtime: encode/decode singular sub-messages (ra6 3d)

Refactor encode_lua/decode_lua into encode_body/decode_body so the
field-walk loop is callable recursively, then dispatch the MESSAGE
kind into a per-side sub-handler. Repeated and map fields still skip
at the field-walk level — 3e (jc9) and 3h (asz) land them next.

encode_submessage_field force-establishes the parent enc_buf's
heap_idx via a no-op ebuf_grow before recursing. Without that the
final ebuf_reserve on the parent could land its new userdata above
sub-encode's leaked stack slots, making the closing lua_settop drop
the parent's heap.

decode_submessage_field bounds the inner read by temporarily
shrinking c->len to the sub-message end offset; the wire-prim
helpers already bounds-check against c->len, so a malformed inner
payload can't over-read into the outer message's bytes.

New fixture test/proto/c_nested.proto carries a 5-level singular
chain (L1->L2->L3->L4->L5) for the depth test. The two existing
"skip message" tests are renamed to "skip repeated and map" — sub-
messages now encode and decode end-to-end.

bd-hwe