Offer the join operators SQLite offers

[trueqbit]

Stacked on #1568: based on its branch, so this diff shows only the changes on top of it.

Summary

SQLite's grammar combines join operators and constraints freely: [NATURAL] [LEFT [OUTER] | RIGHT [OUTER] | FULL [OUTER] | INNER | CROSS] JOIN, followed by ON expr, USING (columns), or nothing (and nothing for NATURAL). sqlite_orm offered six fixed combinations. #1568 made every join carry a constraint, explicit or implicit, and this PR builds the rest on that.

One generic join node

The six join structs become aliases of one node, join_clause<Keyword, T, Constraint>, as the binary operators are aliases of binary_operator since #1552:

template<class T, class O>
using left_join_t = join_clause<left_join_string, T, O>;

template<class T>
using cross_join_t = join_clause<cross_join_string, T, implicit_join_constraint>;

is_any_join_v now recognizes the node instead of listing six templates. The node names, the factories and the serialization stay as they are. The static tests that name internal::left_join_t<…> keep working through the aliases.

The new join operators

Factory	SQL	Constraint
join<T>(), inner_join<T>(), left_join<T>(), left_outer_join<T>()	as named	implicit; new overloads next to the constrained ones
natural_inner_join<T>(), natural_left_join<T>(), natural_left_outer_join<T>()	NATURAL INNER JOIN, …	implicit, by construction
right_join, right_outer_join, full_join, full_outer_join	RIGHT [OUTER] JOIN, FULL [OUTER] JOIN	ON/USING, or implicit; SQLite 3.39+
natural_right_join, natural_right_outer_join, natural_full_join, natural_full_outer_join	NATURAL RIGHT [OUTER] JOIN, …	implicit; SQLite 3.39+

• NATURAL is spelled with dedicated factories, matching the existing natural_join and left_outer_join. Since they take no constraint, NATURAL can't get an ON or USING.
• Every factory has a C++20 overload taking an alias, like the existing ones.
• RIGHT and FULL are guarded by SQLITE_VERSION_NUMBER >= 3039000.
• TODO.md drops RIGHT/FULL JOIN and the combination entry from Treat every join as constrained, explicitly or implicitly #1568.

CROSS JOIN … ON (which SQLite also accepts) is left out, as it's an inner join in all but name.

Test plan

The first commit adds the tests on their own; the generic node and the new operators follow.

• New tests/statement_serializer_tests/ast/join_operators.cpp: every new operator serializes, with the constraint left out or spelled out, including an aliased NATURAL LEFT JOIN.
• tests/join_tests.cpp: executed against two tables that share the column id, the joins yield the rows SQLite defines. That's JOIN and LEFT JOIN without a constraint, NATURAL INNER/LEFT/LEFT OUTER, and on SQLite 3.39+ RIGHT JOIN, NATURAL RIGHT JOIN, FULL OUTER JOIN USING and NATURAL FULL JOIN.
• tests/static_tests/node_tuple.cpp: every new operator is a join node, with an implicit constraint where none is spelled out, and left_join<User>() is left_join_t<User, implicit_join_constraint>.
• CI covers the SQLite versions on both sides of 3.39: 3.31.1, 3.34.1 and 3.35.5 without RIGHT/FULL, 3.51.0 with them.

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