feat(terminal): add configurable cursor render delay for terminal buffers

Add cursor position restoration with configurable timing to handle terminal
     buffer rendering asynchronously. This fixes cursor column being reset to 0
     when refreshing jj log commands.

     Changes:
     - Add `terminal.cursor_render_delay` configuration option (default: 10ms)
     - Add buffer.get_cursor() and buffer.set_cursor() helpers in core/buffer
       * buffer.set_cursor() uses defer_fn with configurable delay for terminal buffers
       * Automatic position validation with clamping to buffer bounds
       * Line number clamped to [1, line_count]
       * Column clamped to [0, line_length] based on actual line content
       * Validation happens inside deferred callback to check against final rendered content
     - Extract clamp_cursor_position() helper to eliminate code duplication
     - Refactor terminal module to use new buffer cursor helpers
     - Store and restore cursor position when cycling through log commands
     - Update README with terminal configuration section and example usage

     The delay is necessary because nvim_open_term() + nvim_chan_send() have
     asynchronous rendering in the terminal emulator layer. Setting the cursor
     before rendering completes results in the column being reset to 0. Users
     experiencing issues can increase the delay value if needed.

lua/jj/core/buffer.lua

@@ -297,4 +297,76 @@ function M.start_insert(buf)
 	vim.cmd("startinsert")
 end
 
-return M
+--- Get cursor position for a buffer
+--- @param buf number Buffer handle
+--- @return number[]|nil Cursor position as {line, col} or nil if buffer not visible
+function M.get_cursor(buf)
+	if not vim.api.nvim_buf_is_valid(buf) then
+		return nil
+	end
+
+	local winid = vim.fn.bufwinid(buf)
+	if winid == -1 then
+		return nil
+	end
+
+	return vim.api.nvim_win_get_cursor(winid)
+end
+
+--- Clamp cursor position to valid buffer bounds
+--- @param buf number Buffer handle
+--- @param pos number[] Cursor position as {line, col}
+--- @return number[] Clamped position as {line, col}
+local function clamp_cursor_position(buf, pos)
+	-- Validate and clamp position to buffer bounds
+	local line_count = vim.api.nvim_buf_line_count(buf)
+	local target_line = math.max(1, math.min(pos[1], line_count))
+
+	-- Get the actual line content to validate column
+	local line_content = vim.api.nvim_buf_get_lines(buf, target_line - 1, target_line, false)[1] or ""
+	local max_col = #line_content
+	local target_col = math.max(0, math.min(pos[2], max_col))
+
+	return { target_line, target_col }
+end
+
+--- Set cursor position for a buffer
+--- Automatically validates and clamps position to buffer bounds:
+--- - Line number is clamped to [1, line_count]
+--- - Column is clamped to [0, line_length] based on actual line content
+--- For terminal buffers, uses defer_fn to allow terminal rendering to stabilize
+--- @param buf number Buffer handle
+--- @param pos number[] Cursor position as {line, col} (1-indexed line, 0-indexed column)
+--- @param opts? {delay?: number} Optional delay in ms for terminal buffers (default: 10)
+function M.set_cursor(buf, pos, opts)
+	if not vim.api.nvim_buf_is_valid(buf) then
+		return
+	end
+
+	opts = opts or {}
+	local delay = opts.delay or 10
+
+	local winid = vim.fn.bufwinid(buf)
+	if winid == -1 then
+		return
+	end
+
+	-- For terminal buffers, delay to allow rendering to complete
+	-- Must validate position INSIDE the deferred function for terminal buffers
+	-- because the buffer content may not be stable yet
+	if vim.bo[buf].buftype == "terminal" or delay > 0 then
+		vim.defer_fn(function()
+			if not vim.api.nvim_buf_is_valid(buf) or not vim.api.nvim_win_is_valid(winid) then
+				return
+			end
+
+			local clamped_pos = clamp_cursor_position(buf, pos)
+			vim.api.nvim_win_set_cursor(winid, clamped_pos)
+		end, delay)
+	else
+		local clamped_pos = clamp_cursor_position(buf, pos)
+		vim.api.nvim_win_set_cursor(winid, clamped_pos)
+	end
+end
+
+return M