refactor: modularize display backends and add benchmark runner

- Create engine/display/ package with registry pattern
- Move displays to engine/display/backends/ (terminal, null, websocket, sixel)
- Add DisplayRegistry with auto-discovery
- Add benchmark.py for performance testing effects × displays matrix
- Add mise tasks: benchmark, benchmark-json, benchmark-report
- Update controller to use new display module

mise.toml

@@ -44,6 +44,14 @@ run-client = { run = "mise run run-both & sleep 2 && $(open http://localhost:876
 cmd = "uv run cmdline.py"
 cmd-stats = { run = "uv run cmdline.py -w \"/effects stats\"", depends = ["sync-all"] }
 
+# =====================
+# Benchmark
+# =====================
+
+benchmark = { run = "uv run python -m engine.benchmark", depends = ["sync-all"] }
+benchmark-json = { run = "uv run python -m engine.benchmark --format json --output benchmark.json", depends = ["sync-all"] }
+benchmark-report = { run = "uv run python -m engine.benchmark --output BENCHMARK.md", depends = ["sync-all"] }
+
 # Initialize ntfy topics (warm up before first use)
 topics-init = "curl -s -d 'init' https://ntfy.sh/klubhaus_terminal_mainline_cc_cmd > /dev/null && curl -s -d 'init' https://ntfy.sh/klubhaus_terminal_mainline_cc_resp > /dev/null && curl -s -d 'init' https://ntfy.sh/klubhaus_terminal_mainline > /dev/null"