[texodus]

This PR adds perspective-migrate, which upgrades persisted configs written by any release since 2.0.0 to the current schema. It covers engine ViewConfigs, ViewerConfigs (<perspective-viewer> save() tokens) and WorkspaceConfigs (saveWorkspace() tokens, including the old lumino <perspective-workspace> layouts).

• Each object's own version field decides how it's upgraded. Without one, it falls back to an optional from_version, then to 2.0.0. Migrating the output again changes nothing.
• Error validation lists what was dropped or approximated, and which schema changes were applied.
• First-party plugin payloads are migrated. Third-party plugins can add their own steps through PluginMigration.
• Written in rust so the same implementaiton is portable in all host languages:
  • Rust: the perspective-migrate crate.
  • @perspective-dev/client:
    • Browser: migrate(), after registering the wasm with init_server({wasm32, wasm64, migrate}). In JS it ships as a separate wasm; It's only downloaded on the first migrate() call, and the CDN bundles register it automatically.
    • Node.js: migrate(), with no registration needed.
  • Python:*perspective.migrate().

Breaking

• restore(), addPanel() and restoreWorkspace() reject keys the config format doesn't define. When a rejected config is stamped by another release, the error names that release and points to migrate().
• Removed legacy fallbacks:
  • array-form expressions;
  • the is None / is not None filter operators;
  • the sum not null and pct sum grand total aggregates;
  • fuzzy matching of plugin names;
• view.get_config() now emits aggregate names that table.view() accepts, e.g. last instead of last value.