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_config.c

@@ -0,0 +1,49 @@
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <xkbcommon/xkbcommon.h>
+
+#include "config.h"
+
+int main(void) {
+    uint32_t mods;
+    char *key = NULL;
+
+    assert(config_parse_mods("ctrl+q", &mods, &key) == 0);
+    assert(mods == MOD_CTRL);
+    assert(strcmp(key, "q") == 0);
+    free(key);
+
+    assert(config_parse_mods("space", &mods, &key) == 0);
+    assert(mods == 0);
+    assert(strcmp(key, "space") == 0);
+    free(key);
+
+    assert(config_parse_action("quit") == ACTION_QUIT);
+    assert(config_parse_action("select:glider") == ACTION_SELECT_GLIDER);
+    assert(config_parse_action("nope") == ACTION_NONE);
+
+    config_t cfg = config_defaults();
+    xkb_keysym_t one = xkb_keysym_from_name("1", XKB_KEYSYM_CASE_INSENSITIVE);
+    xkb_keysym_t space = xkb_keysym_from_name("space", XKB_KEYSYM_CASE_INSENSITIVE);
+    assert(config_lookup(&cfg, 0, one) == ACTION_SELECT_GLIDER);
+    assert(config_lookup(&cfg, 0, space) == ACTION_ROTATE);
+
+    const char *path = "/tmp/opencode/test_keys.conf";
+    FILE *fp = fopen(path, "w");
+    assert(fp);
+    fputs("[bindings]\n\"1\" = \"quit\"\n", fp);
+    fclose(fp);
+
+    config_t cfg2 = config_load(path);
+    assert(config_lookup(&cfg2, 0, one) == ACTION_QUIT);
+    assert(config_lookup(&cfg2, 0, space) == ACTION_ROTATE);
+
+    config_free(&cfg);
+    config_free(&cfg2);
+    remove(path);
+    printf("config tests passed\n");
+    return 0;
+}