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_pattern.c

@@ -0,0 +1,28 @@
+#include <assert.h>
+#include <stdio.h>
+
+#include "pattern.h"
+
+int main(void) {
+    assert(pattern_count() == 4);
+    const void *p = pattern_get(0);
+    (void)p;
+    int dx[16], dy[16];
+    int n = pattern_rotated_offsets(0, 0, dx, dy);
+    assert(n == 5);
+
+    int w, h;
+    pattern_bounds(0, 1, &w, &h);
+    assert(w == 3 && h == 3);
+
+    int dx2[16], dy2[16];
+    pattern_rotated_offsets(0, 1, dx2, dy2);
+    int same = 1;
+    for (int i = 0; i < n; i++) {
+        if (dx[i] != dx2[i] || dy[i] != dy2[i]) same = 0;
+    }
+    assert(!same);
+
+    printf("pattern tests passed\n");
+    return 0;
+}