Fix race condition and potential use-after-free on system distro & init port access

[Blue (OneBlue)]

Summary of the Pull Request

This change solves a synchronization issue between a thread accessing a running instance (via CreateLxProcess() for instance) and a thread terminating an instance.

std::shared_ptr isn't actually safe to read & write to at the time, so accesses to the system distro are wrapped in an std::atomic, and m_initChannel is now protected under m_lock

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Copilot review overview

🟢 Approval recommended

The synchronization changes consistently address the identified shared-pointer races without introducing unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes concurrent access to WSL instance resources during termination.

Changes:

• Makes system-distro ownership atomic and uses stable local references.
• Synchronizes init-port retrieval with instance shutdown.

File	Description
WslCoreInstance.h	Defines atomic system-distro ownership.
WslCoreInstance.cpp	Safely loads, clears, and retrieves shared resources.

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