json,text: presence-tracked fields skip proto3 default elision

Both codecs gated default-value elision on `f.optional or f.oneof` —
the proto3 implicit-presence shape. That elides proto2 `required`
fields set to zero and any other presence-tracked descriptor that
doesn't carry the proto3 explicit-optional flag.

Extend the bypass to `f.required` so a Cardinality{r=0} survives
the round-trip through pb.json.encode / pb.text.encode. 4 new
luatest cases pin the rule.

runtime: parser + dynamic accept proto2 sources

Parser now captures `required=true` and `default_value=…` from the
proto2 keywords (instead of dropping `required` silently and ignoring
field options). Dynamic descriptor builder reads `parsed.syntax`,
flips the repeated-scalar packing default for proto2, and calls
`codec.compile_writers/compile_readers` so the per-field
required-writer specialization actually fires — without that the
generic encode_field path silently elides missing required fields.

64-bit-int defaults are coerced into the appropriate cdata type
inside `coerce_default` so the codec's value-comparison rules
match what generated code emits.

Adds 7 dynamic-mode luatest cases including a static-vs-dynamic
byte-parity check. 725 tests pass.

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.