docs: clarify indexed control compaction semantics

[maxatwork]

Summary

• add DOM test coverage for omitted unchecked indexed checkboxes with sparse index compaction
• document that unchecked indexed controls do not reserve compacted array slots
• clarify that row identity should come from another submitted field when it matters

Test Plan

• npm test

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Summary by cubic

Clarifies that omitted indexed controls do not reserve slots during array compaction, and adds a DOM test to validate this behavior. Docs also note to include another submitted field to preserve row identity when needed.

^{Written for commit 355f21d. Summary will update on new commits.}

[cubic-dev-ai]

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