add layouts support

[jossephus]

This PR adds support for predefined layouts. It enables specifying layouts with tabs, panes and floating pane support.

supports

• tabs with children panes and floating panes
• direction for panes - col and row
• title, window ratio, width, height
• cwd, cmd for panes
• visibility and specifying active tab
• default_layout to use for all sessions

depends on: #86 since we are using prise.spawn to spawn a layout with cmd

example layouts config with video.

local config = {
    theme = { ... },
    default_layout = "ide",
    layouts = {
        simple = {
            name = "simple",
            tabs = {
                {
                    title = "Main",
                    root = { type = "pane" },
                    floating = {
                        pane = { type = "pane" },
                        visible = false,
                    },
                },
            },
        },
        monitor = {
            name = "monitor",
            active_tab = 2,
            tabs = {
                { title = "Shell", root = { type = "pane" } },
                { title = "Htop", root = { type = "pane", cmd = "htop" } },
            },
        },
        ide = {
            name = "ide",
            tabs = {
                {
                    title = "Code",
                    root = {
                        type = "split",
                        direction = "row",
                        children = {
                            {
                                type = "pane",
                                ratio = 0.3,
                                cwd = "/Users/jossephus/workspace/zigspace/src/prise",
                                cmd = "nvim AGENTS.md",
                            },
                            {
                                type = "split",
                                direction = "col",
                                ratio = 0.7,
                                children = {
                                    { type = "pane", ratio = 0.7, cmd = "nix develop --verbose" },
                                    { type = "pane", ratio = 0.3 },
                                },
                            },
                        },
                    },
                    floating = {
                        pane = { type = "pane", cwd = "~" },
                        visible = true,
                        width = 120,
                        height = 30,
                    },
                },
                {
                    title = "Logs",
                    root = { type = "pane", cwd = "/var/log" },
                },
            },
        },
    },
}

layouts.mp4

work and config design started with https://ampcode.com/threads/T-019bae2e-2593-7412-8b5e-d760915f9aac

[github-actions]

Layout System Review

Overall this is a solid implementation of the layout feature. The code follows TigerStyle conventions with good assertions at API boundaries, and the security-conscious approach in expand_path() (avoiding shell commands for directory checks) is appreciated.

Issues Found

1. Line 1492: Missing .close existence check before calling pty:close() on floating panes (trivial fix provided)
2. Line 3701: available_height can go negative on small screens, causing unexpected loop behavior (trivial fix provided)
3. Line 2868: Magic number 15 for visible height calculation doesn't match items_start_y = 8 in build_layout_picker() - could cause scrolling inconsistencies

Minor Observations

• The finalize_layout function is ~80 lines, slightly over the TigerStyle 70-line guideline, but the logic is cohesive and well-structured
• Good error recovery pattern: building new state in memory before destroying old state

---

Full review context: https://ampcode.com/threads/T-019bafe4-7a36-732c-aba8-2d453c444ad1

[jossephus]

fixed all three reported issues including the refactoring of finalize_layout