feat(settings): expose public viz settings contracts - #1587

Summary

• Add graphistry.viz_settings as a public facade for canonical visualization settings key contracts.
• Export URL parameter and React-facing setting key constants, name sets, Literal key aliases, typed payload dictionaries, and frontend contract metadata from one stable module.
• Add anchored drift tests proving the public facade matches the canonical frontend owner modules and validators.

Refs #1234

Exported constants/types

• URL params: URL_PARAM_NAMES, URL_PARAM_NAME_SET, URLParamName, URLParamsDict, KnownURLParamsDict
• React settings: REACT_SETTING_NAMES, REACT_SETTING_NAME_SET, ReactSettingName, ReactSettingsDict, KnownReactSettingsDict
• Combined surface: VIZ_SETTING_NAMES, VIZ_SETTING_NAME_SET, VizSettingName
• Apply-encodings React keys: APPLY_ENCODINGS_REACT_KEYS, APPLY_ENCODINGS_REACT_KEY_SET, ApplyEncodingsReactKey, ApplyEncodingsReactSettingsDict
• Contract metadata: GRAPHISTRY_FRONTEND_CONTRACT_VERSION, signature/version info helpers, and shared payload type aliases

LOC buckets

• Production: +214 / -0, net +214 LOC
• Tests: +67 / -0, net +67 LOC
• Docs/changelog: +1 / -0, net +1 LOC

Compiler-plan surface touched

No. This is exported visualization settings typing/key metadata only. It does not touch GFQL compiler-plan semantics, route names, execution dispatch, diagnostics, schema/type-system hooks, IR metadata, remote schema hooks, or compatibility executors.

Coordination

PR #1584 is already merged. Its encode-call validation/types changes do not conflict with this settings-key facade; this PR only re-exports the existing frontend settings contract and adds Literal key aliases.

Validation

• python3 -m pytest -q graphistry/tests/test_viz_settings.py graphistry/tests/test_validate_settings.py -> 22 passed
• ./bin/ruff.sh graphistry/viz_settings.py graphistry/tests/test_viz_settings.py graphistry/__init__.py -> passed
• ./bin/typecheck.sh graphistry/viz_settings.py graphistry/tests/test_viz_settings.py -> passed
• git diff --check -> passed
• DGX/cuDF: skipped; no DataFrame, cuDF, runtime execution, or GPU path touched