CLI/config: YAML round-trip broken, --config discards other flags, and many accepted options are inert

Found by the September 2026 comparison against the C++ sources (see docs/migration-from-cpp.md on branch docs/legacy-heritage (PR #17)). Verified on main.

**Broken**
- `lanet-vi config out.yaml` dumps enum defaults as `!!python/object/apply:lanet_vi.models.config.DecompositionType` tags and `load_config_from_yaml` uses `yaml.safe_load`, so `--config out.yaml` fails with a ConstructorError. (`io/config_loader.py`; use `model_dump(mode="json")`.)
- With `--config`, every other CLI flag except `--weighted`/`--multigraph` is discarded (`cli.py` ~172-178). The C++ precedence was defaults < config file < command line.
- `--names` loads names but never draws labels: `VisualizationConfig.show_node_labels` defaults to False and is not settable from the CLI (`matplotlib_renderer.py` ~274). `--font-zoom` is therefore dead too. A per-node name `"0"` is treated as "skip"; in the C++ `-names` without a file meant "label with node numbers".
- Default-true booleans (`--show-degree-scale`, `--show-size-legend`, `--gradient-edges`, `--color-by-community`, `--draw-community-boundaries`) have no `--no-...` form and cannot be switched off.
- `--show-degree-scale` is mis-mapped: it toggles the colour legend, while the degree/size legend is `--show-size-legend`.

**Accepted but inert** (either wire them or remove them from the CLI/YAML/`config` output):
`--from-layer`, `--coord-distribution`, `--alpha`, `--beta`, `--delta`, `--gamma`, `--use-spiral-layout` + `--spiral-*`, all `--detect-communities`/`--community-*`/`--color-by-community`/`--draw-community-boundaries` (`config.community` is never read and `community_viz.py` has no callers), YAML `renderer` (`networkx`/`plotly`), `measure`, `unit_length`, `ratio_constant`, `use_spatial_hashing`, `opacity` (only used when `gradient_edges` is off). `maximum_strength` is YAML-only.

Start with io/config_loader.py and cli.py around the cited config-loading and flag-handling sections, then inspect matplotlib_renderer.py and community_viz.py for the rendering paths. Compare the current behavior with docs/migration-from-cpp.md and verify which listed options are wired or inert. Done means config precedence, YAML round-tripping, flag mappings, boolean negations, and the accepted-option behavior are corrected or deliberately removed, with tests covering the changes.

Written by the indexing model from the issue text.