Support tuple/composite field access and ARRAY subscripts in expressions

[renecannao]

From corpus testing against CockroachDB's 3,191 parser test queries (36 PARTIAL results):

Tuple field access (PostgreSQL/CockroachDB):

• SELECT (i.keys).col — composite type field access
• SELECT (i.keys).* — composite type wildcard
• SELECT (expr).@N — CockroachDB ordinal access

Tuple literals:

• SELECT (1, 2, 3) — row constructor
• SELECT ((1, 2, 3) AS a, b, c) — labeled tuple

ARRAY subscripts:

• SELECT (ARRAY['a', 'b'])[2] — array element access

These are valid PostgreSQL syntax used by ORMs and application code. Priority: medium.

[renecannao]

Array subscript evaluation implemented: ARRAY[...][n] with 1-based indexing for PostgreSQL, 0-based for MySQL. Parser and emitter were already complete. Field access and tuple construction return null (composite types deferred).