Kanata: Add virtual key press/release variants

[malpern]

Problem

Kanata supports virtual key press and release actions but KeyPath only models tap.

Current State

• Virtual key tap only

Missing

• Virtual key press
• Virtual key release

Impact

Users can't create hold-based virtual key actions.

Fix

1. Model press/release virtual key variants
2. Add UI controls
3. Generate correct syntax

Priority

Low - tap covers most use cases.

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 4