codegen: proto2 baseline — required, optional, custom defaults

Lifts the proto3-only syntax gate in the plugin and threads three
new field-descriptor attributes through codegen and the codec:

  * required=true   — fields declared with the proto2 `required` keyword.
                      Inline codegen and the runtime codec both error when
                      a required field is missing on encode (vs the silent
                      elide that proto3 implicit-presence fields get).
  * optional=true   — already wired for proto3 explicit `optional`; in
                      proto2 every singular field carries it via the
                      existing HasOptionalKeyword() check, giving presence
                      semantics without a separate emission path.
  * default_value=… — proto2 [default = X] from the field descriptor,
                      rendered as a Lua literal (cdata for 64-bit ints,
                      symbolic name for enums) so consumers can surface
                      it; the codec itself does not auto-materialize
                      defaults on decode, matching how proto3 absent
                      fields stay nil.

Packed-by-default already flips correctly because we ask
protoreflect's `IsPacked()`, which is syntax-aware.

Adds test/proto/proto2_basic.proto with 33 luatest cases covering
required validation, optional presence, custom defaults, the proto2
unpacked-by-default repeated rule, nested-required messages, and
full-vs-runtime mode parity. `just gen-proto2-tests` regenerates the
fixture into examples/expected/{full,runtime}/.

Out of scope: extensions, extend, group; conformance harness still
skips TestAllTypesProto2.