Remove Rust code, delegate server to nuwiki-rs repo

Delete all Rust crates (crates/) and Cargo files. The nuwiki-ls binary
is now built in the separate nuwiki-rs repository and downloaded at
install time from the Gitea release assets, with a cargo build fallback
that clones nuwiki-rs.

Update all documentation to reflect the split repo layout.

development/tests/test-calendar.sh

@@ -10,8 +10,8 @@
 #   * opting out with use_calendar = false.
 #
 # A stub calendar-vim on the runtimepath provides :Calendar and seeds the
-# default hook names. The LSP binary is built (setup() registers the client);
-# the calendar paths themselves don't need the server.
+# default hook names. The LSP binary is downloaded (setup() registers the
+# client); the calendar paths themselves don't need the server.
 #
 # Exit: 0 when both runs are green, 1 otherwise. Skips (0) without nvim.
 
@@ -28,13 +28,34 @@ if ! command -v nvim >/dev/null 2>&1; then
   exit 0
 fi
 
-# setup() registers the LSP client, so build the binary like the other
-# Neovim harnesses do (incremental, fast).
+# Download the LSP binary from nuwiki-rs releases (falls back to cargo build).
 BIN="$REPO_ROOT/bin/nuwiki-ls"
-log 'building nuwiki-ls (release, incremental)…'
-cargo build --release -p nuwiki-ls --manifest-path "$REPO_ROOT/Cargo.toml" >/dev/null
-mkdir -p "$REPO_ROOT/bin"
-ln -sf "$REPO_ROOT/target/release/nuwiki-ls" "$BIN"
+log 'downloading nuwiki-ls…'
+if [[ ! -x "$BIN" ]]; then
+  vim -e -s -c "source $REPO_ROOT/scripts/download_bin.vim" -c "q" >/dev/null 2>&1 || true
+fi
+
+# If download failed, fall back to cargo build from nuwiki-rs.
+if [[ ! -x "$BIN" ]]; then
+  log "download failed, falling back to cargo build from nuwiki-rs…"
+  rs_repo="$REPO_ROOT/nuwiki-rs"
+  if [[ ! -f "$rs_repo/Cargo.toml" ]]; then
+    log "cloning nuwiki-rs …"
+    git clone --depth 1 https://code.gfran.co/gffranco/nuwiki-rs.git "$rs_repo"
+  fi
+  cargo build --release -p nuwiki-ls --manifest-path "$rs_repo/Cargo.toml" >/dev/null
+  if [[ ! -f "$rs_repo/target/release/nuwiki-ls" ]]; then
+    echo 'FAIL: could not build nuwiki-ls binary' >&2
+    exit 1
+  fi
+  mkdir -p "$REPO_ROOT/bin"
+  ln -sf "$rs_repo/target/release/nuwiki-ls" "$BIN"
+fi
+
+if [[ ! -x "$BIN" ]]; then
+  echo 'FAIL: could not obtain nuwiki-ls binary' >&2
+  exit 1
+fi
 
 # --- Scratch wiki with one seeded diary entry (2026-05-30) ----------------
 WIKI="$TMP/wiki"