fix: Lineage works with unions

[nightscape]

Fixes lineage tracking for append (UNION) operations to correctly track inputs from both source tables.

Previously, when using append to union two tables, the lineage only tracked the top table's inputs. The append function now merges bottom.inputs into top.inputs (similar to how join handles inputs), ensuring both source tables are tracked.

Added test to verify both inputs are tracked and column-level lineage works correctly.

[nightscape]

The second commit is worth discussing:
If a table is union'd with itself, should lineage track both occurrences?
I have the slightest tendency towards "yes", because it is two different ways to get to the same table, both possibly containing different filters etc.
But if you deem the existing behavior correct, I'll happily adapt the implementation!

[max-sixty]

thanks for the PR @nightscape !

overall looks good!

I'm a bit worried about the "identical tables have very different behavior from almost-identical tables", though. I haven't looked enough in detail, but how much of a step back would this be:

  diff --git a/prqlc/prqlc/src/semantic/resolver/transforms.rs b/prqlc/prqlc/src/semantic/resolver/transforms.rs
  index 5f07a7499c01..xxxxx 100644
  --- a/prqlc/prqlc/src/semantic/resolver/transforms.rs
  +++ b/prqlc/prqlc/src/semantic/resolver/transforms.rs
  @@ -757,14 +757,12 @@ fn append(mut top: Lineage, bottom: Lineage) -> Result<Lineage, Error> {
                       except: except_b,
                   },
               ) => {
  -                // If both are All columns from the same input, merge the except sets
  -                // Otherwise, keep the top's input_id
  -                // Note: In a union, both inputs should be available, so we keep top's
  +                // Merge except sets from both tables
  +                // This preserves exclusion information even when input_ids differ
  +                // (e.g., "from employees select !{name}" append "from managers select !{salary}")
                   let mut except = except_t;
  -                if input_id_t == input_id_b {
  -                    // Same input, merge except sets
  -                    except.extend(except_b);
  -                }
  +                except.extend(except_b);
  +
                   LineageColumn::All {
                       input_id: input_id_t,
                       except,

(for transparency Claude helped me with the diff)

[nightscape]

@max-sixty I applied your proposed change and ran the tests, everything still green 👍
I rebased on the latest main and force pushed.
All good to go from my side!

[max-sixty]

thank you!