feat: validate JOIN ON operands in strict mode

[tiagolauer]

Closes #205.

Problem

Strict mode validated the SELECT list and (since #128/#148) the WHERE clause. Nothing else:

query	before
select id from users where nope = 1	QueryTypeError<'unknown column: nope'>
select u.id from users u join orders o on u.id = o.nope	{ id: number }

The join condition is the highest-traffic place to get a column name wrong, and the usual mistake is a wrong side (o.id where o.user_id was meant) — which compiles, runs, and returns a wrong result set instead of an error.

Fix

ExtractJoinOnText (src/from.ts) collects every top-level ON condition in the FROM clause, joining multiple groups with and. It is depth-tracked, so an ON inside a derived table's own subquery belongs to that subquery's parse, not this one. The collected text goes through the existing WhereClauseError scan — the operands are ordinary bare or qualified column references against the same sources, which ValidateWhereOperand already handles (placeholders and literals short-circuit as before).

ParsedStatement now carries the FROM clause as raw fromText rather than pre-extracted conditions, so the scan is never instantiated for a non-strict query.

GROUP BY / HAVING / ORDER BY

Deliberately still out of scope — they resolve against SELECT-list aliases, ordinals and aggregates, not only source columns. The README said only that unknown columns become a QueryTypeError in strict mode, with no clause boundary; it now states the boundary explicitly, and tests/join-on-strict.test-d.ts locks it in.

Tests

tests/join-on-strict.test-d.ts (16 assertions): unknown column and unknown alias in ON, second join in a chain, LEFT JOIN, compound a = b and c = d conditions, derived-table joins, CROSS JOIN with no ON, placeholders, a WHERE typo alongside a valid ON, non-strict staying permissive, and the three out-of-scope clauses. Five of them fail on master.

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