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).

tests/test_grid.c

@@ -0,0 +1,25 @@
+#include <assert.h>
+#include <stdio.h>
+
+#include "grid.h"
+
+int main(void) {
+    grid_t g;
+    grid_init(&g, 4, 4);
+    assert(grid_get(&g, 0, 0) == 0);
+    grid_set(&g, 1, 2, 1);
+    assert(grid_get(&g, 1, 2) == 1);
+    assert(grid_get(&g, -1, 0) == 0);
+    assert(grid_get(&g, 4, 4) == 0);
+
+    grid_set(&g, 0, 0, 1);
+    grid_resize(&g, 6, 2);
+    assert(grid_get(&g, 0, 0) == 1);
+    assert(grid_get(&g, 1, 2) == 0);
+    grid_clear(&g);
+    assert(grid_get(&g, 0, 0) == 0);
+
+    grid_free(&g);
+    printf("grid tests passed\n");
+    return 0;
+}