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/pattern.h

@@ -0,0 +1,21 @@
+#ifndef PATTERN_H
+#define PATTERN_H
+
+typedef struct { int dx, dy; } cell_offset_t;
+
+typedef struct {
+    const char *name;
+    int count;
+    const cell_offset_t *cells;
+} pattern_t;
+
+int pattern_count(void);
+const pattern_t *pattern_get(int index);
+
+/* Fill out_dx/out_dy with normalized rotated offsets. Returns cell count. */
+int pattern_rotated_offsets(int index, int rotation, int *out_dx, int *out_dy);
+
+/* Bounding box of rotated pattern. */
+void pattern_bounds(int index, int rotation, int *w, int *h);
+
+#endif