feat: fan out multi-column-alias lineage to every alias

[takaebato]

Summary

Implements the last follow-up noted in #55 (design agreed in discussion):
lineage for Spark's multi-column alias expr AS (a, b, …) now fans out to
every alias instead of stopping at the first.

SELECT explode(t.arr) AS (k, v) FROM t
-- before: reads [t.arr],  lineage [arr → k]
-- after:  reads [t.arr],  lineage [arr → k, arr → v]

Design: the fan is one item (no references)

NamedExpr gains names: OutputNames — Single(Option<Ident>) for an
ordinary item, Fan(Vec<Ident>) for a multi-column alias:

SELECT id, explode(t.arr) AS (k, v), name FROM t
Projection.exprs = [                                  ← 3 items, 4 positions
  NamedExpr { names: Single(id),  expr: Column(t.id) }        // position 0
  NamedExpr { names: Fan([k, v]), expr: Call(explode(arr)) }  // positions 1, 2
  NamedExpr { names: Single(name), expr: Column(t.name) }     // position 3
]

• The expression exists exactly once → reads count it once by
  construction (occurrence-based, unchanged), and item walks (own_exprs)
  stay a plain map.
• Names can never separate from their expression — there is no
  back-reference to dangle under any reorder / filter / splice (the design
  review rejected a positional Fanout { back } reference and a
  debug_assert-guarded fallback for exactly this reason).
• Item index ≠ output position: every positional / named consumer goes
  through one slot view (output_slots / slot_count /
  resolved_output_expr), where a fan yields its shared expression once per
  name — query-output lineage, INSERT positional pairing, RETURNING, CTAS /
  CREATE VIEW, the shared positional trace (set-op branches, scalar
  subqueries, EXCLUDED), derived-table name traces, output-column scopes
  (OutputCol, fan outputs are never identity), AS d(x, y) positional
  renames, and the created-columns / insert arity checks.
• Edge kind = the head expression's kind: explode(arr) fans out
  Transformations; a bare column t.a AS (x, y) fans out Passthroughs.

Non-breaking: output shapes are unchanged; lineage gains edges (the
first-alias-only behavior was a documented best-effort in #55, not a
contract).

Verified end-to-end: fan-out for 2 aliases, Passthrough fan-out for a bare
column, the tail alias traced through a derived table, INSERT pairing and
CTAS / CREATE VIEW feeding every target column, positional fan renames through
a derived alias list (AS d(x, y)) and a CTE column list, UNION branches
tracing positionally (existing reads-once pins unchanged). All gates green
(fmt / clippy / test --all / doc); 766 tests, 100% patch coverage
(llvm-cov ∩ diff).

🤖 Generated with Claude Code

[github-actions]

✅ PR title follows the Conventional Commits spec.

[codecov]

Codecov Report

❌ Patch coverage is 98.80952% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.84%. Comparing base (f326d1a) to head (55b4266).

Files with missing lines	Patch %	Lines
sql-insight/src/resolver/binder.rs	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   94.85%   94.84%   -0.01%     
==========================================
  Files          27       27              
  Lines        5737     5747      +10     
  Branches     5737     5747      +10     
==========================================
+ Hits         5442     5451       +9     
  Misses        216      216              
- Partials       79       80       +1     

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