c_runtime: repeated + packed scalar encode/decode (ra6 3e)

Add repeated dispatch to the C-runtime encode/decode loop. Encode
side: encode_repeated_field walks Lua arrays via lua_objlen + per-
index rawgeti, dispatches on element kind. Packed numerics build
their payload in a stack-backed sub-buffer then emit `tag(LEN) +
varint(len) + body`; unpacked emit `tag + value` per element via
encode_one_field with force_emit=1 to bypass zero-suppression;
strings/bytes flow through the same path (never packable); repeated
messages reuse encode_submessage_field per element.

Decode side: per-field stack-slot cache (list_stack_idx[]) tied to
list_count[] avoids the per-element lua_getfield(result, name) round
trip the c-accel spike measured at 2x slower at 100KB. On first hit
for a repeated field we lua_createtable + write result[name] AND dup-
push the list onto the stack; subsequent hits lua_rawseti through the
cached absolute stack index. Lists stay valid across recursive sub-
message decodes because each child decode_body cleans up its own
scratch back to the caller's frame.

Packed/unpacked symmetry on read: a wt==LEN payload for any packable
scalar is decoded as a packed blob regardless of the schema's packed
flag, and a per-element-tagged stream is decoded element-by-element
even on a schema that defaults to packed — per proto3 reader rules.

New test/proto/c_repeated.proto fixture carries packed + explicit-
unpacked + repeated string/bytes + repeated message branches. The
encode and decode tests round-trip at 10/100/1000 elements per
branch. The two existing "skip repeated and map" marker tests
collapse to "skip map" — only map fields remain out of scope for
3e (bd-asz / 3h lands them next). 854 → 896 passing tests.

bd-jc9