[ZhuchkaTriplesix]

From the review of conditions on hot paths. Refs #1550.

Filter (grid_filter_engine.dart, runs once per row)

• A predicate no longer parses the cell as a number unless its target is a number (a text target used to pay trim + double.tryParse on every cell).
• The operator is resolved to an enum when the predicate is built; evaluate no longer walks three string switches per row.
• _FreeTextNode lower-cases its term once, not per row. The substring fallback lower-cases each cell once per row, not once per term.

Grid rows (grid_row_renderer.dart): the selection test is done once per row and once per cell instead of up to six times per cell.

Clipboard (grid_selection_manager.dart)

• toJson: a cheap first-character check before the two number parses, and an allocation-free true/false comparison. Side effect: text such as Infinity / NaN stays text instead of becoming a double that jsonEncode cannot encode.
• CSV/TSV escaping is one pass over the code units instead of four contains scans.

Tests for numeric vs text targets, case-insensitive free text, JSON edge values and CSV/TSV quoting. Not measured; the gains are estimated from the code.