perf(security): memoized token scanner in PiiMasker.detectPiiColumn eliminating dynamic RegExp and split/join #1540

Problem

In PiiMasker.detectPiiColumn (lib/core/security/pii_masker.dart), every column of every executed query triggers:

• Two dynamic regexes in _tokens (r'([a-z0-9])([A-Z])' and r'[^A-Za-z0-9]+').
• Multiple string transformations: replaceAllMapped -> replaceAll -> toLowerCase -> split(' ') -> where -> toList -> join('_').
• A third dynamic regex (RegExp(r'(^|_)(api|private|access|secret|auth)_key($|_)')).
  For 30 columns, this generates over 250 temporary heap objects per query execution.

Solution

• Add a static thread-safe LRU/cache map (_piiCache) since standard database column names (id, name, created_at, status, etc.) are highly repetitive.
• Replace regex transformations with a simple character scanner on ASCII code units (codeUnitAt).

Benefits

• Instant O(1) cache hits for recurring column names.
• Zero regex compilations and zero split/join allocations during PII masking.