Bulletproof handling around the GPIO fd

[fhunleth]

No description provided.

[Copilot]

Pull request overview

This PR hardens GPIO resource lifecycle handling by making “closed/not-open” state explicit via pin->fd = -1, aligning cleanup paths, and ensuring stub backend read/write fail cleanly when the pin isn’t open.

Changes:

• Add pin->fd < 0 guards to the stub backend’s hal_read_gpio/hal_write_gpio to return -EBADF when not open.
• Set pin->fd = -1 in the cdev backend’s hal_close_gpio after closing the fd.
• Make NIF cleanup call hal_close_gpio unconditionally and normalize pin->env reset to NULL; document that hal_close_gpio is idempotent.

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	Prevents stub read/write operations on unopened/closed pins by returning -EBADF.
c_src/hal_cdev_gpio.c	Marks the GPIO fd invalid after close to avoid reuse/accidental operations on a stale fd.
c_src/gpio_nif.h	Documents that hal_close_gpio may be called more than once.
c_src/gpio_nif.c	Simplifies release logic by always calling hal_close_gpio and using NULL for cleared env pointers.

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