feat: Simplex manifest discovery and client configuration

[KyrylR]

Import roots were previously derived from the directory containing the open file, with unstable compiler features hardcoded to UnstableFeatures::all(). Neither matched how simplex actually builds a package, and neither was configurable.

Adds a project model (src/project.rs) and a settings model (src/config.rs), and wires both into parse_program:

• Finds the nearest Simplex.toml (or simplex.toml) in the file's ancestors, honours build.src_dir, and resolves [dependencies] recursively, including transitive path dependencies and git dependencies installed under deps/-.
• Dependency mappings are context-scoped, so a dependency's own imports resolve against its package rather than the root one.
• Client settings arrive via initialization_options or workspace/didChangeConfiguration, accepted both wrapped in a simplicityhl key (VS Code) and as the bare section.
• Manual project.dependencies supplement the manifest and override it on a colliding alias/context pair.
• experimentalFeatures.imports now drives UnstableFeature::Imports rather than enabling every unstable feature unconditionally.
• Project errors, such as a configured manifestPath that does not exist, surface as diagnostics instead of being swallowed.
• Configuration, workspace-folder and watched-file changes re-analyse the open documents.

Not implemented:

• ProjectContext::dependency_aliases - only import-path completion uses it, which lands with the multi-file analysis.

[gerau]

ACK ca91e0f; code review, tested locally with vscode extension