Add join variants and examples

[dzmitry-lahoda]

Notes

Based on #2

Assuming it is not possible to make rows_number nice enough to use #5

Summary

Adds join and positional pairing variants to OQL:

• join_must: inner equality join that panics when an outer row has no matching inner row.
• join_left: left equality join that keeps unmatched outer rows and projects joined fields as Option values.
• last_must: fast must-match join that keeps only the last inner row per key, avoiding per-key match vectors when only the final match is needed.
• zip: positional pairing that matches Rust Iterator::zip and stops when either side ends.
• zip_must: positional pairing that panics if the two sources have different lengths.

Also adds regression coverage for left-join field rewriting through let bindings and nested method receivers, plus dedicated zip.rs tests, docs, and UI snapshot updates.

Validation

• cargo fmt
• cargo test -p oql --test zip
• cargo test -p oql

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