Клиент: начальный импорт кодовой базы

Основа — Horizon (MIT). История апстрима не переносится: этот репозиторий ведёт свою.

.github/pull_request_template.md

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+## What and why
+
+<!-- What this changes, and why. Link the issue it fixes ("Fixes #123"). -->
+
+## Vanilla reference
+
+<!-- The vanilla classes and methods this follows (tools/rea_vanilla.py), and the harness that checks it.
+     "Horizon-only" for a feature vanilla doesn't have; "none" for tooling or docs. -->
+
+## Self-check
+
+<!-- Paste the block `python3 tools/pr_ready.py` prints (`--base upstream/main` from a fork). -->
+
+## Screenshots
+
+<!-- For anything players see: before and after. Delete this section otherwise. -->
+
+<!-- Please tick "Allow edits by maintainers": we push small fixups ourselves instead of a round trip.
+     See CONTRIBUTING.md. -->