feat: implement keyboard and mouse input, pattern placement, and HiDPI scaling

Add configurable key bindings (TOML), modifier-aware dispatch, mouse-driven
pattern placement with preview and rotation, cell grid storage/rendering,
and fractional/integer HiDPI scaling. Archive the input-and-placement change
and sync its specs to main specs.

Tests: Hashtable, Config, Grid, Pattern (4/4 pass).

include/grid.h

@@ -0,0 +1,18 @@
+#ifndef GRID_H
+#define GRID_H
+
+typedef struct {
+    int cols;
+    int rows;
+    unsigned char *cells;
+    int show_lines;
+} grid_t;
+
+void grid_init(grid_t *g, int cols, int rows);
+void grid_free(grid_t *g);
+void grid_resize(grid_t *g, int new_cols, int new_rows);
+void grid_clear(grid_t *g);
+int grid_get(const grid_t *g, int c, int r);
+void grid_set(grid_t *g, int c, int r, int val);
+
+#endif