docs: document why Owner::own_result poisoning is safe

[QuantumExplorer]

Summary

• Document in Owner::own_result why the poisoning behavior is not exploitable and does not need to be fixed
• Replaces the closed PR fix: eliminate Owner poisoning in own_result by requiring value recovery on error #621 which attempted to change the signature to force value recovery

Context

A security audit flagged that Owner::own_result leaves the Owner in a poisoned state when the closure returns Err. Investigation confirmed this is not exploitable:

• All call sites are in Walker methods (walk, walk_expect, attach, put_value, etc.)
• On every error path, the Walker (which owns the Owner<TreeNode>) is dropped immediately
• The poisoned Owner is never accessed after own_result returns Err
• The panic is unreachable in practice

Changing the signature to FnOnce(T) -> Result<T, (T, E)> was evaluated and rejected — it adds complexity to every closure for no practical benefit.

This PR adds an "audit note" section to the doc comment so future audits don't re-investigate or attempt to fix this non-issue.

Test plan

• Existing tests pass (no code changes, only doc comment)

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9254ab5a-4826-4c18-8829-afd3fdbc5579

📥 Commits

Reviewing files that changed from the base of the PR and between 650acf4 and 129f113.

📒 Files selected for processing (1)

• merk/src/owner.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch doc/owner-poisoning-not-exploitable

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