codegen: int64_as_number opt-in flag for 64-bit decode as Lua number (5y9)

Adds a plugin generator option:

    protoc --tarantool_opt=int64_as_number=true ...

mode=full only, default off, error on mode=runtime. When set, 64-bit
scalar decoders (int64/uint64/sint64/fixed64/sfixed64) return a Lua
number for values that fit [-2^53, 2^53] (inclusive — both endpoints
are powers of two and exact as doubles), cdata otherwise. The decoded
type becomes value-dependent under this option; `+`/`-`/`*`/`==`
work transparently on both, so most callers don't have to care.

Wire side: decode_int64_n / decode_uint64_n / decode_sint64_n /
decode_fixed64_n / decode_sfixed64_n added alongside the existing
decoders. Bounds use LL/ULL cdata literals so the threshold compares
compile to plain 64-bit integer compares on a hot trace.

Codegen side: cfg.Int64AsNumber threads to writer.int64AsNumber;
decodeFnSuffix() emits "_n" only when the flag is set and the scalar
is one of the affected kinds. Every wire.decode_<st> emit site picks
it up — singular, repeated, packed, oneof, extension, map value.

The flag is a no-op under PB_ENABLE_C=1: the C runtime decides
number-vs-cdata on its own via luaL_pushint64 (Tarantool's small-fits-
in-double convention). The test gates on PB_ENABLE_C accordingly.

Workload-specific tradeoff measured on c_int64.Wide decode (full mode,
no PB_ENABLE_C, 5 fields per message):
  tiny   (all 1-byte vars)   2050 -> 1700 ns/op   (-17%, cdata avoided)
  medium (3-byte vars)       3220 -> 3600 ns/op   (+11%, extra cmp+tonumber)
  huge   (past 2^53)         7575 -> 7750 ns/op   (+2%, noise)

Enable when fields are dominated by small IDs/counters/small
timestamps that hit the 1-byte varint path; leave off otherwise.
The plugin flag help documents the tradeoff.

Test fixture: examples/expected/full_n/c_int64/c_int64_pb.lua via
the new gen-int64-as-number Justfile recipe. test/int64_as_number_test.lua
covers byte-identical encoding, small-value Lua number returns, default
cdata returns, 2^53 boundary inclusive, past-2^53 cdata fallback, and
Lua-number-input round-trip.

Suites: test 771/771, test-c 1057/1057 (5 skipped under PB_ENABLE_C=1).

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.