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/config.h

@@ -0,0 +1,59 @@
+#ifndef CONFIG_H
+#define CONFIG_H
+
+#include <stdint.h>
+#include <xkbcommon/xkbcommon.h>
+
+#define MOD_CTRL  (1u << 0)
+#define MOD_SHIFT (1u << 1)
+#define MOD_ALT   (1u << 2)
+#define MOD_SUPER (1u << 3)
+
+typedef enum {
+    ACTION_NONE = 0,
+    ACTION_QUIT,
+    ACTION_CANCEL,
+    ACTION_ROTATE,
+    ACTION_SELECT_GLIDER,
+    ACTION_SELECT_BLINKER,
+    ACTION_SELECT_BLOCK,
+    ACTION_SELECT_TOAD,
+    ACTION_CLEAR,
+    ACTION_PAUSE,
+    ACTION_STEP,
+    ACTION_SPEED_UP,
+    ACTION_SPEED_DOWN,
+    ACTION_TOGGLE_GRID,
+} action_t;
+
+typedef struct {
+    uint32_t mods;
+    xkb_keysym_t keysym;
+    action_t action;
+} binding_t;
+
+typedef struct {
+    binding_t *items;
+    int count;
+    int capacity;
+} config_t;
+
+/* Built-in defaults. */
+config_t config_defaults(void);
+
+/* Ensure config file exists; create with defaults if missing. Returns 0 on success. */
+int config_ensure_default_file(const char *path);
+
+/* Load config from path, merging over defaults. */
+config_t config_load(const char *path);
+
+void config_free(config_t *cfg);
+
+/* Lookup action for a key press. Returns ACTION_NONE if no binding. */
+action_t config_lookup(const config_t *cfg, uint32_t mods, xkb_keysym_t keysym);
+
+/* Helpers exposed for tests. */
+int config_parse_mods(const char *s, uint32_t *out_mods, char **out_key);
+action_t config_parse_action(const char *s);
+
+#endif