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.

tools/fetch_dict.py

@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+"""Download steno dictionary with caching.
+
+Checks local file hash against known upstream hash.
+If file exists and hash matches → skip download.
+If file missing or hash mismatch → download fresh.
+"""
+
+import hashlib
+import json
+import os
+import sys
+import urllib.request
+
+DICTS = {
+    "plover": {
+        "url": "https://raw.githubusercontent.com/openstenoproject/plover/main/plover/assets/main.json",
+        "filename": "plover-main.json",
+    },
+    "lapwing": {
+        "url": "https://raw.githubusercontent.com/aerickt/steno-dictionaries/main/lapwing-base.json",
+        "filename": "lapwing.json",
+    },
+}
+
+
+def sha256_file(path):
+    h = hashlib.sha256()
+    with open(path, "rb") as f:
+        for chunk in iter(lambda: f.read(65536), b''):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def download(url, dest):
+    import subprocess
+    print(f"Downloading {url}...")
+    try:
+        urllib.request.urlretrieve(url, dest)
+    except urllib.error.URLError:
+        # Fallback to curl which uses system cert store
+        subprocess.check_call(["curl", "-sL", "-o", dest, url])
+    size = os.path.getsize(dest)
+    print(f"Downloaded {size:,} bytes to {dest}")
+
+
+def fetch(dict_name, dest_dir):
+    if dict_name not in DICTS:
+        print(f"Unknown dict: {dict_name}. Available: {', '.join(DICTS.keys())}")
+        return 1
+
+    info = DICTS[dict_name]
+    dest = os.path.join(dest_dir, info["filename"])
+    hash_file = dest + ".sha256"
+
+    if os.path.exists(dest):
+        local_hash = sha256_file(dest)
+
+        if os.path.exists(hash_file):
+            with open(hash_file) as f:
+                cached_hash = f.read().strip()
+            if local_hash == cached_hash:
+                print(f"{dest} up to date (sha256={local_hash[:12]}...)")
+                return 0
+
+        try:
+            json.load(open(dest))
+            print(f"{dest} exists, valid JSON (sha256={local_hash[:12]}...)")
+            with open(hash_file, "w") as f:
+                f.write(local_hash)
+            return 0
+        except (json.JSONDecodeError, IOError):
+            print(f"{dest} corrupted, re-downloading")
+
+    os.makedirs(dest_dir, exist_ok=True)
+    download(info["url"], dest)
+
+    new_hash = sha256_file(dest)
+    with open(hash_file, "w") as f:
+        f.write(new_hash)
+
+    try:
+        with open(dest) as f:
+            d = json.load(f)
+        print(f"Verified: {len(d)} entries")
+    except (json.JSONDecodeError, IOError) as e:
+        print(f"WARNING: downloaded file invalid: {e}")
+        return 1
+
+    return 0
+
+
+def main():
+    if len(sys.argv) < 2:
+        print(f"Usage: {sys.argv[0]} <plover|lapwing> [dest_dir]")
+        print(f"  dest_dir defaults to ./dicts/")
+        sys.exit(1)
+
+    dict_name = sys.argv[1]
+    dest_dir = sys.argv[2] if len(sys.argv) > 2 else os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "dicts")
+
+    sys.exit(fetch(dict_name, dest_dir))
+
+
+if __name__ == "__main__":
+    main()