fix(plugin): scope diagnostics to the first branch of a set operation

[tiagolauer]

Fixes #294.

The bug

select id from users union select id from posts
       ^^ ambiguous column: id

Since UNION branches must be column-compatible, sharing column names is the normal case — which makes this a squiggle on nearly every UNION. The shape type-locked in tests/union.test-d.ts reproduces it identically, and the core types the query cleanly (first-branch shape, per the README).

The fix

findSources regex-scans every from/join in the whole statement into one scope, and the select-list check counts the first SELECT's columns against that pooled scope, reporting anything found twice as ambiguous.

The issue offers per-branch scopes or skipping UNION statements entirely. This does the narrower thing that keeps diagnostics working: stop at the first top-level set operator. The first branch is exactly the branch the core takes the row shape from, so what is diagnosed matches what is typed, and later branches fall under the plugin's stated "not reported on, instead of reported wrongly" policy.

Truncating rather than rescoping matters for correctness of the spans: every offset before the cut is unchanged, so no position arithmetic in this file had to move.

The scan is paren-depth aware, so a union inside a subquery does not cut the statement short.

Verification

ts-plugin/tests/diagnostics.test.ts, five new cases (two red on master — PASS (43) FAIL (2) with the src change stashed):

• select id from users union select id from posts reports nothing
• same for union all, intersect, except
• a typo in the first branch is still reported (unknown column: nope)
• a genuine ambiguity inside the first branch is still reported (users join posts, then a union)
• a union inside a subquery does not truncate the statement (the outer typo is still found)

Full plugin suite passes; tsc --noEmit -p ts-plugin/tsconfig.json clean.