fix(tables): cycle Tab into the next row instead of inserting one

Pressing <Tab> past the last cell unconditionally appended a new row
below, even when there was already a next row to jump into. Tabbing
through a header → separator → body table from the header row
inserted a blank row between header and separator instead of landing
in the first body cell.

After the final cell, walk forward from the current row to the end
of the table, skip any separator row, and jump to the first cell of
the next data row. Only insert a fresh row when the cursor is on the
table's last row (or only separator rows follow). Applied to both
the Vim (autoload/nuwiki/commands.vim) and Neovim (lua/nuwiki/
commands.lua) sides so the two harnesses stay in sync; covered by
new keymap tests in scripts/test-keymaps-vim.vim and test-keymaps.lua.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/test-keymaps.lua

@@ -454,6 +454,21 @@ vim.defer_fn(function()
     expect_lines = { '| a |x b |' },
     wait_ms = 100,
   })
+  run('tab.cycles_to_next_row_when_not_last', {
+    lines = { '| a | b |', '|---|---|', '| c | d |' },
+    cursor = { 1, 6 }, -- inside last cell of header row
+    keys = 'i<Tab>x<Esc>',
+    -- Past the final cell of row 1: skip separator, land in first cell of row 3.
+    expect_lines = { '| a | b |', '|---|---|', '|x c | d |' },
+    wait_ms = 100,
+  })
+  run('tab.inserts_row_only_when_on_last_row', {
+    lines = { '| a | b |', '|---|---|', '| c | d |' },
+    cursor = { 3, 6 }, -- inside last cell of last data row
+    keys = 'i<Tab>x<Esc>',
+    expect_lines = { '| a | b |', '|---|---|', '| c | d |', '|x   |   |' },
+    wait_ms = 100,
+  })
   run('shift_tab.prev_cell_in_table', {
     lines = { '| a | b |' },
     cursor = { 1, 6 }, -- inside cell `b`