feat(CompareBoard): add board constants and storage schema

src/features/CompareBoard/model/const/const.ts

@@ -0,0 +1,35 @@
+/**
+ * localStorage key for the persisted board (pairings + focal + specimen).
+ */
+export const BOARD_STORAGE_KEY = 'glyphdiff:board';
+
+/**
+ * Per-role typography storage key — header AdjustTypography instance.
+ */
+export const HEADER_TYPO_KEY = 'glyphdiff:typo:header';
+
+/**
+ * Per-role typography storage key — body AdjustTypography instance.
+ */
+export const BODY_TYPO_KEY = 'glyphdiff:typo:body';
+
+/**
+ * Schema version stamped into persisted board state (gates future
+ * migrations / the URL share-state codec).
+ */
+export const BOARD_SCHEMA_VERSION = 1;
+
+/**
+ * Hard cap on side-by-side columns that still preserve an honest measure.
+ */
+export const MAX_COLUMNS = 3;
+
+/**
+ * Default shared specimen — one header line + one body paragraph (single
+ * language). Used to seed the board and as the share-state fallback.
+ */
+export const DEFAULT_SPECIMEN = {
+    header: 'The Art of Harmonious Type',
+    body:
+        'Good typography is invisible. It guides the eye without calling attention to itself, balancing rhythm, contrast, and proportion so the reader forgets there is a typeface at all and simply reads.',
+};