chore(map-editor): bump to v0.3.1; cheatsheet + README sync

Bumps the module version to v0.3.1 (lib-core.maps dep pin was
already bumped to 0.5.7 in CT6 to allow the editor to load).
Updates the file header, startup banner, cheatsheet Tab entry
(now mentions all three modes), and appends a Cell-Tile Mode
subsection to the README.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -131,3 +131,25 @@ future engine + lib slice.
 All previous hotkeys (Tab, S, E, R, H, 0, D, G, I, Esc) continue to
 work; their bindings are documented in the items where applicable
 and in the in-editor Cheatsheet modal.
+
+## Cell-Tile Mode (v0.3.1)
+
+Third painting mechanic alongside Auto-Tile and Direct. LMB paints a
+per-cell material flag at the cursor cell; RMB clears it. The
+renderer uses the classical 47-blob 8-neighbour bitmask for
+cell-tile cells, unlocking the 9 atlas slots that Auto-Tile's
+dual-grid 4-corner rule structurally cannot reach (isolated, end,
+corner_open, straight, tee_open, tee_half, cross_open, cross_q1,
+cross_q2adj).
+
+Auto-Tile (vertex-paint, 5 reachable slots, dual-grid look) and
+Direct (manual override placement, all 14 slots via explicit pick)
+are unchanged. Tab cycles through all three modes.
+
+For clean visuals, use a single painting mode per layer. Mixing
+Vertex-Tile and Cell-Tile material in the same layer is allowed but
+produces an asymmetric seam at the boundary because vertex-tile
+cells are structurally blind to their cell-tile neighbours.
+
+See `sporel-meta/docs/superpowers/specs/2026-06-01-map-editor-cell-tile-mode-design.md`
+for the full design rationale and the Boris-taxonomy background.