fix: add safety guards to remove() in Simple and Normal

[Koan-Bot]

Summary

• Simple::remove(): Add lock check before removing — attempting to remove an unlocked element now raises a clear error instead of silently corrupting state
• Normal::remove(): Add retry limit (10) to the lock contention loop — prevents infinite loops if another process repeatedly re-locks a contested element

Motivation

Both issues are edge cases around concurrent access:

1. Calling remove() on an unlocked element in Simple.pm would unlink the data file without checking for a lock, potentially leaving orphaned state
2. The while(1) loop in Normal.pm's remove() could spin indefinitely under pathological lock contention

Test plan

• Existing test suite passes (237 tests)
• Manual verification of error messages with invalid states

Supersedes #7 (which was polluted with unrelated commits from fork infrastructure).

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