use advisory file lock for the RGB runtime lock

[berserker-systems]

Replace the presence-based lockfile (created on acquire, removed on drop) with an OS advisory lock held for the runtime's lifetime. The kernel releases it automatically when the process dies, so a crash/kill no longer leaves a stale lock that must be deleted on start, which also restored proper mutual exclusion.

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.71%. Comparing base (1e6353d) to head (22715d0).

Files with missing lines	Patch %	Lines
src/utils.rs	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   95.74%   95.71%   -0.03%     
==========================================
  Files          23       23              
  Lines       12835    12848      +13     
==========================================
+ Hits        12289    12298       +9     
- Misses        546      550       +4     

Flag	Coverage Δ
rust	95.71% <96.42%> (-0.03%)	⬇️

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[zoedberg]

Thanks for this PR. We're currently a bit swamped, so we won't be able to review this right away. We plan to dive into it in a couple of weeks and will follow up then. Really appreciate your patience!

[berserker-systems]

@zoedberg
No worries and thank you for the notice.

As explanation: Android and iOS use SIGKILL sometimes to terminate a process. This will leave a stale lockfile.
At the moment, we remove it manually when the process starts. In my opinion this PR is the correct approach to handle the situation.