[andrewzolotukhin]

Description

deepSort introduced in #254 can duplicate an existing record when its input contains a hole: map preserves the hole, sorting moves the populated entry, and writing the result back leaves the original populated slot in place. Validate every position with Array.from so a missing record rejects before sorting or mutation.

Original request

Provide reusable, documented deep helpers with a unit-test safety net. This follows up on a sparse-input case found while verifying the newly merged sorter.

What changed

• Visit every input position during key validation, including holes.
• Add a regression proving TypeError, preservation of the hole and preservation of the original record reference.
• Document dense-input requirements and include a patch changeset.

Reasoning

The existing validation-before-mutation contract should apply to sparse arrays as well. The fix preserves ordinary stable sorting and the public interface.

Type of Change

• Bug fix
• Documentation update

Blog post

Skipped: correctness fix in a non-visual data-processing helper.

Screenshots / preview evidence

Not applicable: pure data helper. A before/after reproduction demonstrates duplication on the original implementation and rejection without mutation after the fix.

Validation

• npm run lint, npm run build, npm run test: pass.
• Full suite: 300 files, 5,381 tests, no type errors.
• npm run test:coverage: all published-package floors pass; the sorter retains 100% lines/statements/functions/branches.
• All required CI passes: lint/build/tests, PostgreSQL, Garage, browser/API/telemetry E2E and Striff.

Checklist

• Added a failing-before/passing-after regression
• Ran lint
• Ran build and test
• Added a changeset