Phase DS-1: TPC-DS Dictionary Encoding — 50 columns across 14 tables

Apply Arrow dictionary(int8(), utf8()) encoding to low/medium-cardinality TPC-DS
columns, following TPC-H Phase 3.3 pattern (+57% throughput improvement).

Encoded columns (50 total):
- customer_demographics: cd_gender, cd_marital_status, cd_education_status, cd_credit_rating
- customer_address: ca_location_type, ca_state, ca_country, ca_street_type
- time_dim: t_am_pm, t_shift, t_sub_shift, t_meal_time
- date_dim: d_day_name
- item: i_category, i_size, i_color, i_units, i_container
- call_center: cc_class, cc_hours, cc_name + address fields (cc_state, cc_country, cc_street_type)
- catalog_page: cp_department, cp_type
- web_page: wp_type
- web_site: web_class + address fields (web_state, web_country, web_street_type)
- warehouse: address fields (w_state, w_country, w_street_type)
- ship_mode: sm_type, sm_code, sm_carrier
- store: s_hours, s_geography_class, s_division_name, s_company_name + address fields (s_state, s_country, s_street_type)
- customer: c_salutation
- promotion: p_purpose

Implementation:
- src/tpcds_main.cpp: DICTIONARY type handling in create_builders() and finish_batch()
- include/tpch/dsdgen_converter.hpp: get_dict_for_field() declaration
- src/dsdgen/dsdgen_converter.cpp: 25+ encode functions + dictionary registry (41 entries)
- src/dsdgen/dsdgen_wrapper.cpp: 9 table schemas updated to dict8

All 24 TPC-DS tables validated (SF=1). Expected: +50-60% performance gain at scale (from Phase 3.3 precedent).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: tsafin

include/tpch/dsdgen_converter.hpp

@@ -192,4 +192,9 @@ void append_dsdgen_row_to_builders(
     const void* row,
     std::map<std::string, std::shared_ptr<arrow::ArrayBuilder>>& builders);
 
+/**
+ * Returns static dictionary Arrow array for dict8-encoded columns, or nullptr.
+ */
+std::shared_ptr<arrow::Array> get_dict_for_field(const std::string& field_name);
+
 }  // namespace tpcds