Preserve AS in CAST expressions during normalization

[joelmarcotte]

Summary

• Preserve the AS type separator in PostgreSQL CAST(expression AS type), including spaced and nested casts.
• Continue removing projection and table aliases, including aliases inside a scalar subquery in a cast.
• Reserve a small amount of extra builder capacity for short statements so the corrected output does not add an allocation in the simple case.

Stacked on #112 (joel.marcotte/cte-normalization).

Validation

• go test ./... -count=1
• go vet ./...
• git diff --check
• Focused benchmark on Apple M5 Pro, Go 1.27.1, seven samples: simple cast 485 to 531 ns/op (+9.4%), nested cast 777 to 847 ns/op (+9.0%), no-cast control 656 to 675 ns/op (+2.9%). Host load varied across runs, so timings are approximate. All focused cases remain at 6 allocs/op; the simple cast uses 352 B/op versus 320 B/op because the output retains AS integer.
• Existing BenchmarkNormalizer samples for Escaping, Grouping, and Complex retained their baseline bytes/op and allocs/op.
• Allocation profile shows no allocation in the CAST context. A forced-GC probe of three rounds of 500,000 CTE/cast normalizations had heap deltas of +16.6 KB during warmup, then -1.4 KB and -1.6 KB.

Compatibility

Normalized CAST output now includes AS type; query signatures derived from it may change. There are no dependency or public API changes.