Kanata: Add deftemplate and deflayermap support

[malpern]

Problem

Kanata supports code generation (deftemplate) and pattern layers (deflayermap) but KeyPath doesn't expose these features.

Missing Features

• deftemplate: Code generation for repetitive config
• deflayermap: Pattern-based layer syntax
• Dynamic conditionals (cond-if)

Impact

Advanced users can't use Kanata's more powerful abstraction features.

Priority

Low - these are power-user features for complex configs.

Fix

1. Model these constructs in Swift config types
2. Consider read-only preservation first (like MAL-36)
3. Add UI later for creation/editing

References

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