fix: harden REPL/terminal code-sending paths

- lua/keymap/repl.lua
  - Replace non-existent `:SlimeSendCurrentLine` with synchronous
    `vim.fn["slime#send"](line .. "\n")` fallback.
  - Support both modern `r.send` and older `r.run` R.nvim Lua APIs,
    falling back to `<Plug>` mappings if neither module is available.

- lua/nix_smart_send.lua
  - Refactor `send_repl()` to extract Tree-sitter node text and send it
    directly via `slime#send`, eliminating the `feedkeys(..., "x", true)`
    race between visual selection and cursor movement.
  - Replace internal `slime#send_op` call in `slime_send_region()` with the
    public `<Plug>SlimeRegionSend` mapping.
  - Improve `move_to_next_non_empty_line()` to walk up the AST when a
    node has no next sibling, so the cursor escapes nested blocks.

- plugin/03_terminal.lua
  - Set `vim.g.slime_target = "neovim"` at the top of the module so the
    target is guaranteed before any slime send.

- plugin/21_datascience.lua
  - Remove duplicate `vim.g.slime_target = "neovim"` now that the
    terminal module owns the setting.

lua/keymap/repl.lua

@@ -20,12 +20,20 @@ function M.send_line()
   local ft = dispatch_ft()
 
   if ft == "r" then
-    -- R.nvim v1+ exposes a Lua API; fall back to the legacy <Plug> mappings
-    -- if a v0.x build is still in use.
-    local ok, rrun = pcall(require, "r.run")
-    if ok and rrun and type(rrun.send_line) == "function" then
-      rrun.send_line()
-      return
+    -- R.nvim v1+ exposes a Lua API; try the modern `r.send` module first,
+    -- then fall back to the older `r.run` module, and finally to <Plug>.
+    local ok, rmod = pcall(require, "r.send")
+    if not ok or not rmod then
+      ok, rmod = pcall(require, "r.run")
+    end
+    if ok and rmod then
+      if type(rmod.line) == "function" then
+        rmod.line()
+        return
+      elseif type(rmod.send_line) == "function" then
+        rmod.send_line()
+        return
+      end
     end
     vim.api.nvim_feedkeys(
       vim.api.nvim_replace_termcodes("<Plug>RDSendLine", true, false, true),
@@ -48,7 +56,8 @@ function M.send_line()
   end
 
   -- Default: vim-slime (terminal).
-  vim.cmd("SlimeSendCurrentLine")
+  local line = vim.api.nvim_get_current_line()
+  vim.fn["slime#send"](line .. "\n")
   -- Move to the next line, matching the previous behaviour.
   vim.cmd("normal! j")
 end
@@ -58,11 +67,20 @@ function M.send_selection()
   local ft = dispatch_ft()
 
   if ft == "r" then
-    -- Prefer R.nvim v1+ Lua API; fall back to <Plug> if unavailable.
-    local ok, rrun = pcall(require, "r.run")
-    if ok and rrun and type(rrun.send_selection) == "function" then
-      rrun.send_selection()
-      return
+    -- Prefer R.nvim v1+ Lua API; try the modern `r.send` module first,
+    -- then fall back to the older `r.run` module, and finally to <Plug>.
+    local ok, rmod = pcall(require, "r.send")
+    if not ok or not rmod then
+      ok, rmod = pcall(require, "r.run")
+    end
+    if ok and rmod then
+      if type(rmod.selection) == "function" then
+        rmod.selection()
+        return
+      elseif type(rmod.send_selection) == "function" then
+        rmod.send_selection()
+        return
+      end
     end
     vim.api.nvim_feedkeys(
       vim.api.nvim_replace_termcodes("<Plug>RSendSelection", true, false, true),