[workstonedai-collab]

Fixes #914.

Problem

Table.lookup() checked only whether an index covered the lookup columns. A non-unique index could therefore be mistaken for the required uniqueness constraint, and a partial unique index could be treated as equivalent to a full unique index. This could allow duplicate lookup rows.

Behavior

• Before: an ordinary or partial index could prevent lookup() from creating the required full unique index.
• After: lookup() reuses an existing full unique index on exactly the lookup columns, or creates one when needed.
• Existing indexes are preserved.
• If existing data violates the required uniqueness constraint, the unique-index creation fails with sqlite3.IntegrityError before any rows are changed.

This change is limited to Table.lookup() unique-index detection and creation. It makes no broad performance or indexing claims and is independent of #913; both are based on the same upstream commit.

Validation

Validation was run by Codex:

• Existing lookup tests, new regression tests, and three standalone probes: 18 passed.
• Repository lookup tests: 15 passed after the fix (4 failed before the fix).
• Full suite: 1502 passed, 16 skipped, 33 warnings.
• mypy: 10 source files passed.
• Black, flake8, and diff checks passed.

Implementation and tests: MiniMax. Review and validation: Codex.

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📚 Documentation preview 📚: https://sqlite-utils--915.org.readthedocs.build/en/915/