docs: clarify security boundaries

[sivajisj]

Summary

Clarifies the distinction between Anchor security boundaries and
defense-in-depth / anti-footgun protections in the security policy.

This makes it clearer which types of issues may represent a security
boundary violation and helps reporters determine the appropriate
reporting path.

Changes

• Define what constitutes a security-boundary issue.
• Identify examples of fundamental Anchor security properties.
• Clarify that not every Anchor safety check represents a security boundary.
• Explain how uncertain security reports should be handled.

Branch Target

This change is documentation-only and does not contain breaking changes,
so it targets the current master branch.

Closes #4857

[vercel]

@sivajisj is attempting to deploy a commit to the OtterSec Team on Vercel.

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