feat: add batch read support via UA_Client_Service_read

[juljimm]

Implement read_node_values to read multiple OPC-UA nodes in a single request, reducing N round-trips to 1. Tested against real PLC: 17.6x speedup (916ms -> 52ms for 36 nodes).

Description

Adds a new read_node_values/2 function that reads the 'value' attribute of multiple OPC-UA nodes in a single service call using UA_Client_Service_read, instead of issuing one request per node.

Motivation: When reading many nodes (e.g. 36 PLC tags), sequential reads cause significant latency due to per-request round-trip overhead. Batching all reads into one OPC-UA Read Service call eliminates this overhead.

Changes:

• src/common.c: New handle_read_node_values() C NIF that builds a UA_ReadRequest with multiple UA_ReadValueId entries and calls UA_Client_Service_read. Results are encoded per-node as {:ok, value} or {:error, status}.
• src/common.h: Declaration for handle_read_node_values.
• src/opc_ua_client.c: Registered "read_node_values" in the client request handler table.
• lib/opc_ua/common.ex: New read_node_values/2 public API, handle_call clause, and C response handler with parse_batch_value/1.
• test/client_tests/batch_read_test.exs: 5 test cases covering multiple values, nil/unset values, nonexistent nodes, single node, and mixed data types.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• 5 unit tests in test/client_tests/batch_read_test.exs (batch read multiple values, nil values, nonexistent node per-node error, single node, mixed data types)
• Manual testing against a real Siemens S7-1500 PLC reading 36 nodes: 17.6x speedup (916ms sequential → 52ms batched)

Test Configuration:

• Toolchain: Elixir/OTP with open62541 v1.4.x C library
• SDK: opex62541

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

---

Note

Medium Risk
Introduces new C port handler and custom response encoding/decoding, so bugs could cause incorrect values or crashes under malformed inputs or large batches; scope is additive and gated to client reads.

Overview
Adds a new read_node_values/2 client API to batch-read the Value attribute for multiple nodes in one OPC-UA request, returning per-node success/error tuples and handling nil/array/structured variant decoding in Elixir.

Implements the C-side handle_read_node_values handler using UA_Client_Service_read, registers it in the client request handler table, exposes the new handler in headers, and adds an ExUnit suite covering multi-node reads, unset values, mixed types, and per-node errors.

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