feat!(terminal): unify terminal window behavior and centralize sizing config (#102)

- add terminal.window.type support (hsplit, vsplit, floating, tab)
- add terminal.window.split_size for split-based layouts
- rename floating size options to terminal.window.floating_width/floating_height
- validate terminal window ratios during setup
- make terminal.run/run_floating merge default keymaps internally
- remove cmd.split width/height sizing in favor of terminal.window config
- improve floating/log buffer state handling and refresh behavior
- update README examples and configuration docs

BREAKING CHANGE: terminal sizing/config moved to terminal.window.*.
Removed cmd.split.width/cmd.split.height.
Renamed terminal.window.width/height to terminal.window.floating_width/floating_height

lua/jj/core/buffer.lua

@@ -39,6 +39,7 @@ local M = {}
 --- @field on_exit? fun(buf: number) Callback when buffer is closed
 --- @field keymaps? jj.core.buffer.keymap[] Keymaps to set on the buffer
 --- @field win_options? table Window-specific options to set
+--- @field zindex? number Stacking order (default: 50)
 
 --- Create and configure a new buffer
 --- @param opts jj.core.buffer.opts Buffer configuration options
@@ -153,6 +154,7 @@ function M.create_float(opts)
 		height = height,
 		row = row,
 		col = col,
+		zindex = opts.zindex,
 		style = opts.style or "minimal",
 		border = opts.border or "rounded",
 	}