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/_common.sh

@@ -10,7 +10,7 @@
 # Exports:
 #   REPO_ROOT / DEV_DIR / WIKI_DIR  — common paths
 #   log                             — prefixed status line
-#   ensure_binary                   — build + symlink nuwiki-ls
+#   ensure_binary                   — verify nuwiki-ls exists (download if missing)
 #   write_sample_wiki DIR           — lay down the feature-showcase wiki
 #   reset_dirs DIR...               — wipe + recreate owned state dirs
 #   seed_wiki                       — (re)seed WIKI_DIR fresh each run; never
@@ -27,20 +27,34 @@ log() { printf '\033[1;34m[nuwiki-dev]\033[0m %s\n' "$*"; }
 
 ensure_binary() {
   local bin="$REPO_ROOT/bin/nuwiki-ls"
-  local built="$REPO_ROOT/target/release/nuwiki-ls"
-  if [[ "${NUWIKI_DEV_SKIP_BUILD:-0}" == "1" ]]; then
-    if [[ ! -x "$bin" ]]; then
-      log "NUWIKI_DEV_SKIP_BUILD=1 but $bin is missing — building anyway"
-    else
-      log "NUWIKI_DEV_SKIP_BUILD=1 — using existing $bin"
-      return
-    fi
+  if [[ -x "$bin" ]]; then
+    log "binary ready: $bin"
+    return
+  fi
+  if [[ "${NUWIKI_DEV_SKIP_BUILD:-0}" == "1" ]]; then
+    log "NUWIKI_DEV_SKIP_BUILD=1 but $bin is missing"
+    return 1
+  fi
+  log "downloading nuwiki-ls…"
+  vim -e -s -c "source $REPO_ROOT/scripts/download_bin.vim" -c "q"
+  if [[ -x "$bin" ]]; then
+    log "binary ready: $bin"
+    return
+  fi
+  log "download failed, falling back to cargo build from nuwiki-rs…"
+  local rs_repo="$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 "$rs_repo/Cargo.toml"
+  if [[ ! -f "$rs_repo/target/release/nuwiki-ls" ]]; then
+    log "cargo build failed"
+    return 1
   fi
-  log "building nuwiki-ls (release)…"
-  cargo build --release -p nuwiki-ls --manifest-path "$REPO_ROOT/Cargo.toml"
   mkdir -p "$REPO_ROOT/bin"
-  ln -sf "$built" "$bin"
-  log "binary ready: $bin (built $(date -r "$built" '+%Y-%m-%d %H:%M:%S'))"
+  ln -sf "$rs_repo/target/release/nuwiki-ls" "$bin"
+  log "binary ready: $bin"
 }
 
 # write_sample_wiki DIR — lay down a fixture that exercises every vimwiki