Rebrand to ImpLab and split debugger into DAP/Widget

[ejgallego]

Summary:

• rename top-level Lean library/namespace from Dap to ImpLab
• move sources into ImpLab/Lang and ImpLab/Debugger/{Core,Session,DAP,Widget}
• keep DAP-facing identifiers where requested (toydap, dap-export, dap-tests, lean-toy-dap, .dap/)
• update docs, VS Code tasks/launch config, and client copy to reflect ImpLab naming
• align stdio adapter namespace with layout (ImpLab.Debugger.DAP.run)

Validation:

• lake build
• lake exe dap-tests
• cd client && npm run compile

Notes:

• no compatibility shim was added; this is a direct refactor for prototype velocity.