Add CLI options for deterministic window geometry and readiness

[lambdalisue]

Summary

Add deterministic window bootstrap options to the Arto CLI so users can reliably control initial window geometry, theme, and capture timing.

Motivation

Arto already supports per-invocation open behavior (--open) and root directory control (--directory).
For automation workflows (screen capture, demos, regression repros, scripted QA), we still lack a stable way to:

1. place a window at an exact position,
2. set an exact window size,
3. force a deterministic theme,
4. know when the initial render is actually ready.

This currently forces fragile AppleScript timing and ad-hoc retries.

Proposed CLI Interface

--position=<x>,<y>

Set initial window position in screen coordinates.

Examples:

• arto --position=120,120 README.md
• arto --open=new --position=80,64 docs/guide.md

--size=<w>,<h>

Set initial window size.

Examples:

• arto --size=1400,920 README.md
• arto --position=120,120 --size=1400,920 README.md

--theme=<light|dark|system>

Override theme for the invocation.

Examples:

• arto --theme=light README.md
• arto --open=new --theme=dark docs/guide.md

--wait-ready

Wait until the target window completes its initial content render before returning control.

Examples:

• arto --wait-ready README.md
• arto --open=new --position=120,120 --size=1400,920 --theme=light --wait-ready README.md

Behavior Notes

• Window target selection (new vs reuse) is decided by --open behavior, not by whether file paths are present.
• Options should apply to the selected target window for that invocation.
• --wait-ready should use a concrete readiness signal (not fixed sleep), e.g. after initial markdown render pipeline completion.
• These options should work correctly through single-instance IPC (secondary instance -> primary instance).
• No-path invocation: when no files/directories are provided, Arto should still follow --open semantics:
  • --open=new: create a new window, then apply geometry/theme options.
  • --open=screen / default behavior: select/reuse the target window, then apply geometry/theme options.

Scope

• Applies to CLI invocations that open files/directories and no-path invocations used for window control.
• Includes argument parsing, IPC protocol propagation, and main-window creation/selection path updates.
• Includes arto --help documentation updates.

Acceptance Criteria

• --position correctly sets target window position
• --size correctly sets target window size
• --position and --size can be combined in one invocation
• --theme overrides theme for the invocation (light / dark / system)
• --wait-ready returns only after initial render is ready
• With no path, --open still determines create-vs-reuse behavior
• Secondary-instance invocations forward these options through IPC correctly
• arto --help documents all new options and value formats