feat: add configurable Host Text actions

[IgorArkhipov]

Addresses https://t.me/c/1464748383/37027/132011

Summary:

• Add five configurable, persisted Host Text actions for arbitrary UTF-8 text, covering Eastern and Western European characters.
• Allow assignment to physical keys, Tap Dance including Hold, key overrides, combos, and layouts.
• Route Host Text through macOS, Windows, X11, IBus, and Fcitx5. .entlayout v4 preserves action configuration and remaps platform transport slots; v1-v3 layouts retain existing Host Text configuration.

Validation:

• cargo test --bin entropy -- --quiet: 329 passed.
• Python syntax check for linux/ibus/entropy-ibus-engine passed.
• git diff --check passed.

Post-merge validation:

• Configure a multi-character non-ASCII action, for example ąćęłńóśźż, then trigger it from a normal key and Tap Dance Hold on each available backend.
• Healthy: exact text inserts once, text expansion does not rerun, and Universal Symbols still output normally.

[IgorArkhipov]