Parameter system hardening: API namespace cleanup + multi-shell completion coverage

[carldebilly]

Summary

This PR completes the parameter-system wave with two main goals:

1. Finalize API surface organization by moving domain-specific public contracts out of the root namespace.
2. Strengthen shell-completion coverage so recent parameter features are checked across all supported shells.

What changed

Public API organization

Moved public contracts into domain namespaces/folders to keep the root namespace focused on high-level types:

• Repl.Parameters
• Repl.Documentation
• Repl.ShellCompletion
• Repl.Terminal
• Repl.Interaction
• Repl.Autocomplete
• Repl.Rendering

Updated project-level global usings where needed (Repl.Core, Repl.Defaults, Repl.Testing, Repl.WebSocket, Repl.Telnet, test projects, and relevant samples) to preserve readability and reduce local using churn.

Completion test coverage

Extended shell completion integration tests to validate key parameter/completion behaviors across all supported shells:

• bash
• powershell
• zsh
• fish
• nu

Coverage now explicitly checks:

• command option suggestions (--verbose, etc.)
• named value option suggestion (--label)
• global option suggestions (--no-interactive, --no-logo)
• enum value suggestions after option token (Fast, Slow)

Why

• Keep the public API cleaner and easier to navigate as feature count grows.
• Ensure new parameter/completion capabilities are consistently validated per shell bridge, not only on a single shell path.
• Reduce regressions where completion behavior can drift per shell adapter/output format.