Kanata: Add advanced one-shot variants support

[malpern]

Problem

Kanata supports 5 types of one-shot behavior but KeyPath only exposes 1 type.

Current State

Basic one-shot support only.

Missing Variants

• one-shot-press
• one-shot-release
• one-shot-press-pcancel
• one-shot-release-pcancel

Impact

Power users can't configure advanced one-shot behaviors via GUI.

Fix

1. Model all one-shot variants
2. Add UI controls for variant selection
3. Generate correct Kanata syntax

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 4
• Kanata docs: one-shot variants