feat(grpc): Node-side @sentrix/chain/grpc client + bundled .proto

[satyakwok]

Summary

Completes the SDK surface coverage. Existing doors: `evm` (viem), `native` (REST), `bft` (WebSocket subs), `wallet` (signing). New: `grpc` — typed wrapper around the chain's `sentrix.v1.Sentrix` service over `@grpc/grpc-js`. Node-only; browser consumers use the gRPC-Web transport in sentrix-explorer-v2 or wire `grpc-web` npm directly.

Why a thin wrapper

1. Bundle the `.proto` with the npm package — version-locked to the SDK so a chain proto bump can't silently mismatch your client.
2. Centralise the endpoint URL (`grpc.sentrixchain.com:443` / `grpc-testnet.sentrixchain.com:443`) via the same network spec the rest of the SDK uses.
3. Hide the `proto-loader` → service-stub plumbing so consumers don't reach into `@grpc/grpc-js` internals.

Available calls (chain v0.4+)

Method	Returns
`getLatestBlock()` / `getBlockByHeight(h)`	`Block`
`getBalance(address)`	`Account` (20-byte hex or Buffer accepted)
`getValidatorSet({ atHeight? })`	`ValidatorSet` (full active set + flags)
`getSupply({ atHeight? })`	`Supply` (minted/burned/circ)
`getMempool({ limit })`	`Mempool` (size + headers)
`streamEvents([filters])`	server-stream of `ChainEvent`

Older chain hosts (v0.2 / v0.3) return `Status::unimplemented` for the newer methods; the SDK forwards the error verbatim so callers can fall back to JSON-RPC / REST.

Usage

```ts
import { GrpcClient } from "@sentrix/chain/grpc";

const c = new GrpcClient("mainnet");
const block = await c.getLatestBlock();
const bal = await c.getBalance("0x4693b113e523A196d9579333c4ab8358e2656553");

// Server-stream — drain with for-await
for await (const ev of c.streamEvents([])) {
console.log(ev);
}

c.close();
```

Build flow

`tsc` compiles src → dist; an inline `mkdir -p dist/grpc-proto && cp` step ships the bundled `.proto` next to the compiled JS so `proto-loader`'s runtime path resolution works for both `pnpm dev` (src/) and the published package (dist/).

Verification

End-to-end against live `grpc.sentrixchain.com:443` — `getLatestBlock()` returns block 1678214 in ~250 ms.

Out of scope

• Browser gRPC-Web wrapper — sentrix-explorer-v2 has the existing tonic-web-wasm-client pattern; extracting it into a `@sentrix/chain/grpc-web` subpath is its own PR.
• Typed message types (Block, Account, etc.) — proto-loader returns loose objects; consumers cast at the use site. A `buf generate` codegen pipeline would emit precise TS types but adds a build dep; revisit when consumers ask.