Cap Linux support

[richiemcilroy]

Add initial Linux support to enable compilation and screen/audio recording.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

In general, any pointer returned from XGetWindowProperty must only be dereferenced after confirming that: (1) the call succeeded, (2) the returned pointer is non‑null, (3) the reported number of items is sufficient for the intended read, and (4) the property type/format matches what we expect for the target Rust type. After using the data, we must call XFree exactly once and never dereference the pointer again.

For this specific code, _NET_WM_PID is defined as a CARDINAL (32‑bit) property, so we should verify that at == x11::xlib::XA_CARDINAL and af == 32, and that ni >= 1, before treating pid_prop as a pointer to a u32. We should also convert the raw pointer into a Rust slice using std::slice::from_raw_parts, which both makes the length explicit and avoids manually dereferencing an arbitrary pointer. A robust change, without altering observable behavior, is therefore:

• Change the dereference block to a nested if that:
  • checks st == 0, !pid_prop.is_null(), ni >= 1, at == x11::xlib::XA_CARDINAL, and af == 32;
  • uses std::slice::from_raw_parts(pid_prop as *const u32, ni as usize) to create a slice;
  • reads slice[0] as the PID;
  • always calls XFree once when pid_prop is non‑null, regardless of whether we ended up using the value.
• Ensure we don’t use pid_prop after it has been freed.

These changes are entirely localized to the let pid = { ... } block around lines 608–635 in crates/scap-targets/src/platform/linux.rs and require no new imports or helper functions.