refactor(config): replace line-level right blocks with layout.rprompt

[lemtoc]

As of v0.3.1 (9e938b4).

Summary

layout.line1.right and layout.line2.right look symmetric but have different rendering semantics. The input-line right block becomes zsh native RPROMPT, while layout.line1.right in a two-line layout is emulated by baking enough spaces into PROMPT to reach COLUMNS.

That emulated first-line alignment makes the prompt width-dependent and is the root cause described in #44: after a pane split or terminal shrink, zsh redisplays a multiline prompt containing padding computed for the previous width.

Proposed fix

Make the right prompt independent of the configured line count:

[layout]
lines = 2
rprompt = ["time"]

[layout.line1]
left = ["dir", "git_branch", "git_status"]

[layout.line2]
left = ["exit_status", "prompt_char"]

• layout.rprompt always lowers to the shell-native input-line RPROMPT for both one-line and two-line layouts.
• Prompt line tables contain left-side segments only.
• Remove first-line padding and its dedicated fitting path from the renderer.
• Move the default time segment and all examples to layout.rprompt.
• Accept legacy line-level right keys for one compatibility release, normalize them into layout.rprompt, and emit an actionable deprecation warning.
• Reject or clearly diagnose ambiguous configurations that mix the new and legacy forms.
• Update configuration, renderer, worker, CLI, snapshot, and documentation coverage.

Acceptance criteria

• One-line and two-line layouts both use zsh native RPROMPT.
• No rendered first line is padded to the terminal width.
• Existing line-level right configurations receive a deterministic migration path.
• Pane split behavior is re-tested after this change before deciding whether fix(zsh): prompt is not re-rendered on terminal resize, so baked right-side padding wraps #44 needs additional resize handling.

Notes

This is a prerequisite and root-cause simplification for #44. It intentionally does not add TRAPWINCH, terminal-specific semantic markers, or preprompt rendering.