feat(core): centralize command execution and add standalone command runner

- add CommandExecutor to unify shared command execution lifecycle
  across Falyx and standalone command execution
- add CommandRunner for running a single Command directly as a CLI
  or programmatic entrypoint
- add Command.build() factory and rename parse_args() to resolve_args()
  to clarify the parsing-to-execution boundary
- introduce ExecutionOption and wire execution-scoped flags into
  CommandArgumentParser and Command construction
- refactor Falyx to use FalyxParser/ParseResult and CommandExecutor
  instead of the older argparse-based flow and run_key path
- simplify __main__.py bootstrap by building a bootstrap Falyx instance
  directly and running flx.run()
- improve completer support for preview commands and unique-prefix
  command resolution
- default BottomBar toggle namespace to "default"
- expand module/class docstrings to reflect the new execution architecture

tests/test_falyx_parser/test_root_options.py

@@ -0,0 +1,47 @@
+from falyx import Falyx
+from falyx.parser.falyx_parser import FalyxParser, RootOptions
+
+
+def get_falyx_parser():
+    falyx = Falyx()
+    return FalyxParser(falyx=falyx)
+
+
+def test_parse_root_options_empty():
+    parser = get_falyx_parser()
+    opts, remaining = parser._parse_root_options([])
+    assert opts == RootOptions()
+    assert remaining == []
+
+
+def test_parse_root_options_consumes_known_leading_flags():
+    parser = get_falyx_parser()
+    opts, remaining = parser._parse_root_options(
+        ["--verbose", "--never-prompt", "deploy", "--env", "prod"]
+    )
+    assert opts.verbose is True
+    assert opts.never_prompt is True
+    assert remaining == ["deploy", "--env", "prod"]
+
+
+def test_parse_root_options_stops_at_first_non_root_token():
+    parser = get_falyx_parser()
+    opts, remaining = parser._parse_root_options(["deploy", "--verbose"])
+    assert opts == RootOptions()
+    assert remaining == ["deploy", "--verbose"]
+
+
+def test_parse_root_options_supports_help():
+    parser = get_falyx_parser()
+    opts, remaining = parser._parse_root_options(["--help"])
+    assert opts.help is True
+    assert remaining == []
+
+
+def test_parse_root_options_supports_double_dash_separator():
+    parser = get_falyx_parser()
+    opts, remaining = parser._parse_root_options(
+        ["--verbose", "--", "deploy", "--verbose"]
+    )
+    assert opts.verbose is True
+    assert remaining == ["deploy", "--verbose"]