Optimize vertex/edge field access with direct array indexing

NOTE: This PR was created using AI tools and a human.

Leverage deterministic key ordering from uniqueify_agtype_object() to
access vertex/edge fields in O(1) instead of O(log n) binary search.

Fields are sorted by key length, giving fixed positions:
- Vertex: id(0), label(1), properties(2)
- Edge: id(0), label(1), end_id(2), start_id(3), properties(4)

Changes:
- Add field index constants and accessor macros to agtype.h
- Update age_id(), age_start_id(), age_end_id(), age_label(),
  age_properties() to use direct field access
- Add fill_agtype_value_no_copy() for read-only scalar extraction
  without memory allocation
- Add compare_agtype_scalar_containers() fast path for scalar comparison
- Update hash_agtype_value(), equals_agtype_scalar_value(), and
  compare_agtype_scalar_values() to use direct field access macros
- Add fast path in get_one_agtype_from_variadic_args() bypassing
  extract_variadic_args() for single argument case
- Add comprehensive regression test (30 tests)

Performance impact: Improves ORDER BY, hash joins, aggregations, and
Cypher functions (id, start_id, end_id, label, properties) on vertices
and edges.

All previous regression tests were not impacted.
Additional regression test added to enhance coverage.

modified:   Makefile
new file:   regress/expected/direct_field_access.out
new file:   regress/sql/direct_field_access.sql
modified:   src/backend/utils/adt/agtype.c
modified:   src/backend/utils/adt/agtype_util.c
modified:   src/include/utils/agtype.h

Author: jrgemignani

Makefile

@@ -112,7 +112,8 @@ REGRESS = scan \
           name_validation \
           jsonb_operators \
           list_comprehension \
-          map_projection
+          map_projection \
+          direct_field_access
 
 ifneq ($(EXTRA_TESTS),)
   REGRESS += $(EXTRA_TESTS)