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.

Initial commit: protoc-gen-tarantool plugin + pb runtime

A protoc plugin (Go) and a pure-Lua + LuaJIT-FFI runtime that give
Tarantool a complete proto3 + gRPC stack. Two codegen modes (full
inline / runtime descriptor), 226-test luatest suite, 18-fixture
mainline-protoc interop corpus, JSON codec, well-known types,
gRPC client/server factories, runtime .proto parser, microbench
harness with allocation regression gate.

Covers PLAN.md M1-M5. Module is `pb` (not `protobuf`) to avoid
colliding with Tarantool's built-in encode-only `protobuf` module.