fix: correctness bugs found during code review

[freeformz]

Summary

• Iter2: Move counter variable inside returned closure so the index resets to 0 when the iterator is reused, instead of continuing from the previous invocation's final value
• Locked.Clone / LockedOrdered.Clone: Remove outer RLock that caused recursive read-lock deadlock under write contention (the inner Iterator call already acquires RLock)
• Ordered.UnmarshalJSON: Use Add() in a loop instead of direct slice assignment, so duplicate values in JSON input are deduplicated rather than corrupting internal state (len(values) != len(idx))
• Chunk: Panic early with a clear message when n <= 0 instead of hitting a cryptic division-by-zero inside the iterator
• SyncMap.Clear: Delete elements during Range instead of counting first then calling Clear() separately, so the returned count accurately reflects what was removed

Test plan

• Added TestIter2ReusedIteratorResetsIndex to verify index resets on iterator reuse
• All existing tests pass with -race flag

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes five correctness bugs discovered during code review, focusing on iterator behavior, concurrency issues, and data structure invariants.

Changes:

• Fixed Iter2 to reset index counter on each iterator reuse by moving the counter variable inside the returned closure
• Removed recursive read-lock deadlock in Locked.Clone and LockedOrdered.Clone by removing redundant outer RLock calls
• Fixed Ordered.UnmarshalJSON to properly deduplicate JSON arrays with duplicate values using Add() instead of direct slice assignment
• Added early panic with clear error message for invalid Chunk parameters (n <= 0)
• Fixed SyncMap.Clear to delete elements during Range iteration for accurate count instead of separate count-then-clear operations

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
set.go	Moved Iter2 counter variable inside closure for proper reset on reuse; added panic check for Chunk with n <= 0
set_test.go	Added TestIter2ReusedIteratorResetsIndex to verify index counter resets correctly
locked.go	Removed redundant outer RLock from Clone to prevent recursive lock deadlock
locked_ordered.go	Removed redundant outer RLock from Clone to prevent recursive lock deadlock
ordered.go	Changed UnmarshalJSON to use Add() loop instead of direct slice assignment to maintain idx/values invariant
sync.go	Refactored Clear to delete during Range for accurate concurrent count; reformatted var declarations

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