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.

plugin/03_terminal.lua

@@ -1,5 +1,9 @@
 local Config = require('config')
 
+-- vim-slime target: use Neovim's built-in terminal.
+-- Must be set before any slime send happens.
+vim.g.slime_target = "neovim"
+
 local M = {}
 
 -- Configuration

plugin/21_datascience.lua

@@ -22,7 +22,6 @@ end
 
 -- terminal
 later(function()
-  vim.g.slime_target = "neovim"
   vim.g.slime_no_mappings = true
   add("vim-slime")
   vim.g.slime_cell_delimiter = vim.g.slime_cell_delimiter or "# %%"