joaoh82 / rust_sqlite / 26806010656

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feat(sql): support JOIN ... USING / NATURAL / CROSS (SQLR-5) (#158)

PR #99 shipped INNER/LEFT/RIGHT/FULL OUTER JOIN ... ON; the three
related shapes SQLite supports were still rejected as NotImplemented.
This wires all three through the existing nested-loop join driver.

Parser (src/sql/parser/select.rs):
- JoinClause.on: Expr → constraint: JoinConstraintKind (On/Using/Natural).
- USING (cols) is narrowed to a list of column names; NATURAL is carried
  as-is (it needs schemas the parser doesn't have); CROSS JOIN is
  rewritten to ON true at parse time.

Executor (src/sql/executor.rs):
- resolve_join_constraint lowers USING/NATURAL into the synthesized
  `left.col = right.col [AND …]` predicate, schema-aware: the left
  qualifier is picked per column so join chains resolve correctly, and
  NATURAL auto-discovers the shared column names (none ⇒ cross product,
  matching SQLite).
- SELECT * de-duplicates USING/NATURAL columns per the SQLite
  convention — the joined-on column shows once, taking the left copy.

Tests: 8 new executor tests (USING≡ON rows, SELECT* dedup for USING and
NATURAL, NATURAL multi-column AND, NATURAL-without-common-cols cross
product, CROSS cartesian product, LEFT OUTER USING, USING unknown-column
error), replacing the old "returns NotImplemented" test. Docs updated
(supported-sql, sql-engine, roadmap, README, web docs).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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