codec: preserve -0.0 for proto3 float/double scalars

Proto3 default-elision dropped any singular float/double whose value
compared equal to 0 — `-0.0 == 0.0` in IEEE so the `if v ~= 0` guard
silently elided negative zero. The wire bytes for -0 differ from +0
and the TextFormatInput conformance corpus pins that -0 must survive
a round-trip; the failure surfaced as 10 unexpected text-suite
regressions covering FloatFieldNegativeZero (3 spellings × 2 outputs)
and Neg{Float,Double}FieldLargeNegativeExponentParsesAsNegZero
(2 types × 2 outputs).

Sign-bit guard via `1/v == math.huge` (positive zero yields +inf,
negative zero yields -inf). Applied in three layers:
  * runtime/pb/codec.lua: is_default_scalar + the specialized
    monomorphic writer for non-optional numeric scalars
  * runtime/pb/text.lua: is_proto3_default (text encoder elision)
  * inline codegen: scalarNotDefaultExpr emits the guard for
    float/double fields in full-mode `_encode` functions

Proto3 text-format conformance now sits at 416 ✓ / 18 skipped / 0
expected failures; binary+JSON holds at 1478 ✓. Unit-test regressions
cover both directions (preserved on encode + decode round-trip, +0
still elided) and use a runtime-computed -0.0 sentinel because LuaJIT
can constant-fold the literal `-0.0` to a sign-less zero in some
load paths.

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.