Fix CTE AS normalization with alias removal

[joelmarcotte]

Summary

• Preserve AS in PostgreSQL CTE headers, including MATERIALIZED and NOT MATERIALIZED forms.
• Continue removing projection and table aliases when alias retention is disabled.
• Cover multiple, recursive, nested, and column-list CTEs.
• Keep up to four CTE frames inline to avoid allocations on common nested queries.

Validation

• go test ./... -count=1
• git diff --check

Performance

On Apple M5 Pro with Go 1.27.1, go test -run '^$' -bench '^BenchmarkCTENormalization$' -benchtime=300ms -count=7 -cpu=1 compared with main:

Query	main	this PR	Allocations
No CTE	656 ns/op	655 ns/op	6 -> 6
Plain CTE	569 ns/op	611 ns/op	6 -> 6
Materialized CTE	805 ns/op	884 ns/op	6 -> 6
Not materialized CTE	708 ns/op	758 ns/op	6 -> 6
Nested CTE	957 ns/op	1,060 ns/op	7 -> 7

The corrected CTE path takes 7–11% longer than the baseline, which discards CTE syntax. The allocation profile attributes no heap allocations to the CTE tracker for these cases. A five-deep CTE exceeds the inline frame capacity and uses one extra allocation.

Compatibility

Affected CTE normalization output changes when alias retention is disabled. Consumers that derive query signatures from normalized SQL may see new signatures for those queries.