Treat '&' as Latin punctuation; add &↔? mapping and tests

[qqrm]

Motivation

• Ensure & is considered Latin punctuation when choosing conversion direction so layout selection behaves correctly for mixed text.
• Support the physical key mapping where & (EN) corresponds to ? (RU) on shifted 7 in the Russian layout.

Description

• Updated is_latin in src/domain/text/mapping.rs to include & as Latin punctuation so it contributes to Latin dominance counting.
• Added reciprocal punctuation mappings in src/domain/text/mapping.rs: ? => '&' in map_ru_to_en and & => '?' in map_en_to_ru to reflect the shifted 7 key behavior.
• Added a new test ampersand_prefers_latin_direction_when_present in src/tests/mapping_invariants_tests.rs that asserts convert_ru_en_bidirectional("a&") == "ф?" and convert_ru_en_bidirectional("я?") == "z&".

Testing

• No automated tests were executed in this rollout; changes include unit tests that will run under the repository test suite (e.g. cargo +nightly test --locked).

---

Codex Task