perf(database): eliminate redundant string and list allocations in postgresResultCellToDisplayString #1521

Summary

In lib/core/database/postgres_result_cells.dart (lines 53–85):

1. Date-Only Formatting:

   if (value is DateTime) {
     if (_isDateOnly(typeOid, dataTypeName)) {
       return value.toIso8601String().split('T').first;
     }
     return value.toIso8601String();
   }

   Calling toIso8601String().split('T').first allocates a full 24-character string, a 2-element list, and the first element. On 100,000 date cells, this creates 200,000 transient objects.
2. Bytea / Binary Hex Encoding:

   String postgresByteaHexDisplay(Uint8List bytes) {
     final out = StringBuffer(r'\x');
     for (final b in bytes) {
       out.write(b.toRadixString(16).padLeft(2, '0'));
     }
     return out.toString();
   }

   b.toRadixString(16).padLeft(2, '0') allocates 2 new String objects per byte. For a 10 KB binary cell, this creates 20,000 String allocations for a single cell!

Proposed Solution

1. Replace toIso8601String().split('T').first with a dedicated date-only formatter:

   static String _formatDateOnly(DateTime d) =>
       '${d.year.toString().padLeft(4, '0')}-${d.month.toString().padLeft(2, '0')}-${d.day.toString().padLeft(2, '0')}';

2. Use a precomputed 256-entry static lookup table for byte-to-hex conversion:

   const _hexTable = ['00', '01', ..., 'fe', 'ff'];
   out.write(_hexTable[b]); // O(1) array lookup with zero string allocations

Acceptance Criteria

• Date-only cells do not call split('T').
• Bytea hex formatting performs zero toRadixString or padLeft allocations.
• Full parity with existing result cell display tests in test/core/database/postgres_result_cells_test.dart.