M6: trace stability gate + two fixes

Add `make jit-trace` (`bench/jit_trace.lua`) — a standalone tarantool
script that attaches a `jit.attach('trace')` listener over each hot
encode/decode path and asserts no aborts in our source files fall into
the fatal set (NYI bytecode, blacklisting, persistent type instability).
Runs outside luatest because on macOS arm64 the test framework exhausts
JIT mcode pages before the test body runs, masking real abort reasons.

Two fixes shipped to make all 13 scenarios pass:

  - `decode_varint` grew a 1-byte fast path. Before, calling it from a
    hot decode loop pulled an inner `while true do` into the caller's
    root trace, which got blacklisted after enough retries.

  - `pb.finalize_message` now precomputes `desc.oneofs_list` (array
    form) and the runtime-mode codec iterates it with ipairs instead
    of `pairs(desc.oneofs)`. `pairs()` over a hash-keyed table compiles
    to bytecode ISNEXT, which is NYI in LuaJIT 2.1.

The gate also reports interpreter-bridge counts as a benchmark-quality
metric. Decoders show 0-4 bridges per run depending on JIT timing —
caused by side traces returning from inlined `decode_varint` calls,
which LuaJIT 2.1 can't stitch back cleanly. Small per-call overhead on
the multi-byte slow path, structural to the engine.

Scope caveat: map fields encode via `pairs()` and remain off-trace —
pinned by the gate's last scenario so we notice if upstream lifts the
restriction.

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.