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.

README.md

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+# zmk-steno-engine
+
+Clean-room steno engine for ZMK keyboards. Split-storage architecture: full dictionary on peripheral (right) half, queried over BLE from central (left) half.
+
+**Status:** Early development — not yet functional.
+
+## Architecture
+
+ZMK splits run two nRF52840 halves connected via BLE. The right (peripheral) half has ~533KB free flash — enough for a full steno dictionary. The left (central) half runs the steno engine.
+
+```
+┌─────────────┐  BLE GATT query  ┌──────────────┐
+│  Left half   │ ───────────────► │  Right half   │
+│  (central)   │ ◄─────────────── │ (peripheral)  │
+│              │   translation     │               │
+│  Steno engine│                  │  Dictionary    │
+│  LRU cache   │                  │  (~533KB)      │
+└─────────────┘                  └──────────────┘
+```
+
+**Key design decisions:**
+
+- Dictionary stored on peripheral → no flash pressure on central
+- BLE GATT custom service for stroke→translation queries
+- LRU cache on central side → reduces BLE round-trips for common words
+- Orthographic rules run on central after dictionary lookup
+
+## License
+
+[PolyForm Noncommercial 1.0.0](LICENSE)