Steno engine with BLE split-storage architecture

Full steno engine: chord detection, MPHF/trie dict lookup, Plover
formatter (18 commands), undo (ring buffer), HID + Unicode output.

Split-storage specific:
- BLE GATT service for dict queries (query/prefix/batch)
- LRU cache on central side (CONFIG_STENO_SPLIT_CACHE_SIZE)
- Dict embedded on peripheral, engine runs on central
- 3-way dispatch: split_dict / MPHF / simple trie

Build system: auto-fetch Plover dict, MPHF compiler with block
compression (~56K entries in 420KB), CMake integration.

include/dt-bindings/zmk/steno_keys.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2024 Afiq Zudin Hadi
+ * SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0
+ *
+ * Standard steno key layout. Each key = bit position in uint32_t chord.
+ * Bit assignments match the compiler's STENO_KEYS bitmask exactly.
+ *
+ * Layout (standard steno order):
+ *   S  T  K  P  W  H  R  A  O  *  E  U  F  R  P  B  L  G  T  S  D  Z  #
+ *   0  1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17 18 19 20 21 22
+ */
+
+#ifndef DT_BINDINGS_ZMK_STENO_KEYS_H_
+#define DT_BINDINGS_ZMK_STENO_KEYS_H_
+
+/* Left hand consonants */
+#define STENO_SL     0   /* S- (left) */
+#define STENO_TL     1   /* T- */
+#define STENO_KL     2   /* K- */
+#define STENO_PL     3   /* P- */
+#define STENO_WL     4   /* W- */
+#define STENO_HL     5   /* H- */
+#define STENO_RL     6   /* R- (left) */
+
+/* Vowels */
+#define STENO_A      7   /* A */
+#define STENO_O      8   /* O */
+
+/* Center */
+#define STENO_STAR   9   /* * */
+
+/* Vowels (right) */
+#define STENO_E     10   /* E */
+#define STENO_U     11   /* U */
+
+/* Right hand consonants */
+#define STENO_FR    12   /* -F */
+#define STENO_RR    13   /* -R (right) */
+#define STENO_PR    14   /* -P (right) */
+#define STENO_BR    15   /* -B */
+#define STENO_LR    16   /* -L */
+#define STENO_GR    17   /* -G */
+#define STENO_TR    18   /* -T (right) */
+#define STENO_SR    19   /* -S (right) */
+#define STENO_DR    20   /* -D */
+#define STENO_ZR    21   /* -Z */
+
+/* Number bar */
+#define STENO_NUM   22   /* # */
+
+#define STENO_KEY_COUNT 23
+
+#endif /* DT_BINDINGS_ZMK_STENO_KEYS_H_ */