feat(split): add jj split command with interactive floating terminal

Add split command that allows splitting a change into multiple revisions
    from the log buffer or via :J split. Supports --parallel, --message,
    --fileset, and --ignore-immutable flags. Includes immutability checks
    with user confirmation and empty change detection.

lua/jj/cmd/split.lua

@@ -0,0 +1,100 @@
+local M = {}
+
+local utils = require("jj.utils")
+local terminal = require("jj.ui.terminal")
+
+--- Clamps a ratio value between 0.1 and 1.0, returning a default of 1.0 if the input is invalid.
+---@param value? number
+---@param field string
+---@return number
+local function clamp_ratio(value, field)
+	if type(value) ~= "number" or value < 0.1 or value > 1.0 then
+		utils.notify(
+			string.format("Value for field `%s` must be between `0.1` and `1.0`. Defaulted to `1.0`", field),
+			vim.log.levels.WARN
+		)
+		return 1.0
+	end
+	return value
+end
+
+local function build_split_command(opts)
+	local args = { "jj", "split" }
+
+	local rev = (opts.rev and opts.rev ~= "") and opts.rev or "@"
+	table.insert(args, "-r")
+	table.insert(args, rev)
+
+	if opts.parallel then
+		table.insert(args, "--parallel")
+	end
+
+	if opts.message then
+		table.insert(args, "--message")
+		table.insert(args, opts.message)
+	end
+
+	if opts.ignore_immutable then
+		table.insert(args, "--ignore-immutable")
+	end
+
+	if opts.filesets then
+		for _, fileset in ipairs(opts.filesets) do
+			table.insert(args, fileset)
+		end
+	end
+
+	return table.concat(args, " ")
+end
+
+--- Split natively
+---@param opts? jj.cmd.split.opts
+function M.split(opts)
+	if not utils.ensure_jj() then
+		return
+	end
+
+	local cmd_mod = require("jj.cmd")
+	opts = vim.tbl_deep_extend("force", cmd_mod.config.split or {}, opts or {}) --[[@as jj.cmd.split.opts]]
+	opts.height = clamp_ratio(opts.height, "height")
+	opts.width = clamp_ratio(opts.width, "width")
+
+	-- If it's empty do nothing
+	if utils.is_change_empty(opts.rev or "@") then
+		utils.notify(string.format("The change `%s` is empty, nothing to split.", opts.rev or "@"), vim.log.levels.INFO)
+		return
+	end
+
+	local function run_split(cmd)
+		terminal.run_floating(cmd, nil, {
+			title = " JJ Split ",
+			modifiable = true,
+			height = math.floor(vim.o.lines * opts.height),
+			width = math.floor(vim.o.columns * opts.width),
+			keep_modifiable = true,
+			interactive = true,
+			on_exit = opts.on_exit or nil,
+		})
+	end
+
+	-- If the change is immutable warn the user and prompt him
+	if utils.is_change_immutable(opts.rev or "@") then
+		vim.ui.select(
+			{ "Yes", "No" },
+			{ prompt = string.format("The change `%s` is IMMUTABLE, do you still want to split it?", opts.rev or "@") },
+			function(item)
+				if item == "Yes" then
+					opts.ignore_immutable = true
+					local cmd = build_split_command(opts)
+					run_split(cmd)
+					return
+				end
+			end
+		)
+	else
+		local cmd = build_split_command(opts)
+		run_split(cmd)
+	end
+end
+
+return M