c_runtime: encode singular scalars (ra6 3b)

New entry pb.c_runtime.encode(plan, msg) -> string. Covers int32/64,
uint32/64, sint32/64, bool, fixed32/64, sfixed32/64, float, double,
enum (number or by_name lookup), string, bytes. Repeated, map, and
message-typed fields are silently skipped — those land in 3d/3e.

Buffer is a 4KB stack scratch promoted to a lua_newuserdata on
overflow, so a mid-encode luaL_error doesn't leak: the userdata is
GC'd at the unwind point.

Proto3 zero-suppression mirrors mode=full byte-for-byte: empty strings,
zero ints/fixed/enum skipped, +0.0 double skipped while -0.0 is emitted
via type-pun u64 equality (matches the `1/v == -math.huge` guard on
the Lua side). Proto3-optional fields bypass suppression. Plans with
desc.encode overrides (WKT) are rejected here — bd-rmf scope.

Tests: 16 cases per codegen mode (full + runtime), covering the bd-y1n
acceptance (Person {name='x', age=42, balance=-7,
user_id=0xDEADBEEFCAFEBABEULL, weight_kg=3.14} byte-equal to mode=full)
plus per-kind sweep and heap-grow path at 8KB. Suite: 806/806 with
PB_ENABLE_C=1, 748/748 + 58 skipped without.

Closes bd-y1n