Scaffolding: ZMK module structure, Kconfig, DTS, behavior driver

- zephyr/module.yml with dts_root for dt-bindings
- Kconfig: STENO_ENGINE, dict selection (Plover/Lapwing/test),
  MPHF toggle, Unicode modes, history size, multi-stroke timeout
- DTS behavior binding (one_param, steno key index)
- behavior_steno.c: chord accumulation, all-up detection,
  multi-stroke buffering with timeout, star undo, formatter pipeline,
  3-way dict dispatch (split/MPHF/simple trie)
- dict_embed.S: .incbin from generated header path
- steno_keys.h: 23-key layout, bit positions matching compiler

dts/behaviors/steno_engine.dtsi

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2024 Afiq Zudin Hadi
+ * SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0
+ */
+
+/ {
+    behaviors {
+        /omit-if-no-ref/ steno: steno_engine {
+            compatible = "zmk,behavior-steno-engine";
+            #binding-cells = <1>;
+        };
+    };
+};