feat(lineage): preserve struct-field path on leaf nodes

[treff7es]

Summary

sqlglot.lineage collapses struct/JSON field access down to the bare column. For

SELECT MIN(widget.metric.a, widget.metric.b) FROM tbl

lineage returns a single leaf tbl.widget — both .metric.a and .metric.b are lost. This affects Snowflake VARIANT, BigQuery STRUCT, Iceberg nested types, and Spark complex types — anywhere users access struct fields.

There are two root causes in to_node:

1. set(find_all_in_scope(select, exp.Column)) dedupes the two value-equal widget columns into one before the leaf loop runs.
2. The leaf is named from c.sql(), which renders only the column reference and drops the exp.Dot ancestor chain that holds the subfield path.

Change

• Add _struct_access_root(column) — climbs the left spine of the exp.Dot chain to the maximal access expression (returns the column unchanged when there is no Dot parent).
• Use it as the dedup key (so t.s.a and t.s.b stay distinct) and as the leaf name (so tbl.widget.metric.a is preserved).

This is the "first-class" representation — the path lives in Node.name, with no new dataclass field and no extra callback.

Results

Query	Before	After
MIN(widget.metric.a, widget.metric.b)	['tbl.widget']	['tbl.widget.metric.a', 'tbl.widget.metric.b']
t.s.a	['t.s']	['t.s.a']
COALESCE(t.s.a, t.s.b)	['t.s']	['t.s.a', 't.s.b']
t.s.a.b.c	['t.s']	['t.s.a.b.c']
plain column a	['tbl.a']	['tbl.a'] (unchanged)

• tests/test_lineage.py: 43/43 pass, no regressions.

Scope / known limitation

Resolves the leaf-level cases (sqlglot#7604). Struct-field access resolved through a CTE / derived table (sqlglot#6258, Example 2) is unaffected — that path recurses with the bare column name and still needs the subfield threaded through to_node recursion. The inline struct(...) construction error (sqlglot#6258, Example 1) is a separate code path and not addressed here.

Refs: tobymao#7604, tobymao#6258

[treff7es]

Superseded by the two-PR split: #3 (leaf-level struct-path preservation) and #4 (struct-field resolution through CTEs / nested constructors / UNNEST, stacked on #3). Review #3 first.