perf(parser): replace input.substring with matchAsPrefix in GridFilterEngine lexer #1512

Summary

In lib/features/workspace/grid_filter_engine.dart (lines 389, 400, 408, 458):

Inside _FilterLexer.tokenize() and _tryMatchPredicate(), logical operators and predicates are checked against a fresh substring copy on every single token iteration:

final rem = input.substring(i);
final andMatch = RegExp(r'^(AND\b|&&)', caseSensitive: false).firstMatch(rem);
...
final rem = input.substring(p);
final isNullMatch = RegExp(r'^IS\s+(NOT\s+)?NULL\b', caseSensitive: false).firstMatch(rem);

Whenever the user types into the data grid filter bar, thousands of transient string slices are allocated, triggering garbage collection and UI stutter.

Architectural Solution: Dedicated Zero-Allocation Micro-Parsers

Decompose _tryMatchPredicate and token matching into dedicated, testable micro-functions operating strictly on (String input, int offset) with zero substring copying:

1. static int? _matchLogicalOperator(String input, int offset)
   • Matches AND, OR, NOT, &&, ||, ! via precompiled static patterns using matchAsPrefix(input, offset).
2. static ({int consumed, _PredicateNode node})? _matchNullPredicate(String input, int start, int colIdx)
   • Micro-parser for IS NULL and IS NOT NULL.
3. static ({int consumed, _PredicateNode node})? _matchInPredicate(String input, int start, int colIdx)
   • Micro-parser for IN (...) and NOT IN (...).
4. static ({int consumed, _PredicateNode node})? _matchBetweenPredicate(String input, int start, int colIdx)
   • Micro-parser for BETWEEN min AND max.
5. static ({int consumed, _PredicateNode node})? _matchComparisonPredicate(String input, int start, int colIdx)
   • Micro-parser for relational operators (=, !=, <, <=, >, >=, :, LIKE, ILIKE).
6. static ({String value, int consumed})? _scanQuotedStringLiteral(String input, int start)
   • Single-pass quote escaper for single ('...) and double ("...) quotes.

Acceptance Criteria

• Each predicate type and operator has an isolated, self-contained micro-function.
• No intermediate input.substring(i) calls in the lexer loop.
• Micro-functions return Dart 3 records with primitive offsets and nodes.
• 100% test pass rate in test/features/workspace/data_grid_filter_engine_test.dart.