fix!: manage external buffer lifetime

[GrapeBaBa]

Motivation

The .external_buffer return hint passed borrowed Zig slices to JavaScript without copying or registering a finalizer. JavaScript can retain the Buffer after the native callback returns, leaving it backed by stack, arena, or otherwise freed memory. External buffers should retain their zero-copy behavior while making ownership and cleanup explicit.

Description

• Add an allocator-backed OwnedBuffer that transfers its allocation to JavaScript.
• Require .external_buffer callbacks to return OwnedBuffer; ordinary byte-slice returns continue to copy.
• Release the backing allocation and finalizer context from the N-API finalizer.
• Match napi-rs ownership semantics: conversion consumes OwnedBuffer; environments that disallow external buffers copy the bytes and release the original allocation, while other errors preserve ownership already accepted by the N-API finalizer.
• Use a normal Buffer for empty data and document the allocator lifetime contract.
• Test the public copy contract and finalizer-context allocation failure, plus a real Node --expose-gc integration test that verifies the external data and finalizer context remain live while reachable and are both released after GC.

[nazarhussain]

This has good test coverage for that tricky case. @GrapeBaBa Good work 👍🏼