Fix memory leak on interrupt notifications

[fhunleth]

It turns out that the message environment when calling enif_send needs
to be cleared. This was found by Claude and is easily confirmed by
watching :erlang.memory(:total) whie processing a lot of interrupts.

[Copilot]

Pull request overview

This PR fixes an Erlang VM memory leak observed while processing high volumes of GPIO interrupt notifications by ensuring the message-building NIF environment used with enif_send is cleared after each send.

Changes:

• Update send_gpio_message to build messages in a dedicated msg_env, copy gpio_spec into that env, and enif_clear_env(msg_env) after sending.
• Adjust the cdev interrupt poller thread to allocate a reusable msg_env once and reuse it across events.
• Update the stub HAL notification path to allocate/free a per-send msg_env around message sending.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
c_src/hal_stub.c	Allocate a message env per notification send and free it afterward.
c_src/hal_cdev_gpio_interrupts.c	Reuse a thread-local message env for interrupt message construction/sending.
c_src/gpio_nif.h	Document and extend send_gpio_message API to accept a message env.
c_src/gpio_nif.c	Build interrupt message terms in msg_env, send via enif_send, and clear msg_env to prevent leaks.

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