test: add proptest property-based tests

Add proptest dependency and property tests for:
- CellKey: key normalization invariants (sort order, dedup, round-trip,
  prefix non-equality, merge commutativity)
- View: axis exclusivity, set_axis, idempotency, page_selection roundtrip,
  hide/show roundtrip, toggle_group_collapse involution

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -22,6 +22,9 @@ flate2 = "1"
 unicode-width = "0.2"
 dirs = "5"
 
+[dev-dependencies]
+proptest = "1"
+
 [profile.release]
 opt-level = 3
 lto = true