Debugger: evaluate/setVariable, exception stops, and coverage expansion

[ejgallego]

Summary:

• add DAP evaluate and setVariable end-to-end (core + stdio adapter)
• add exception-stop flow with setExceptionBreakpoints and exceptionInfo
• expand core and transport tests for frame-selection behavior, negative paths, and exception toggle semantics
• update debugger docs/roadmap and client README with supported requests and caveats

Validation:

• lake build
• lake build toydap
• lake exe dap-tests

Notes:

• evaluate expression parsing is intentionally minimal for now (variable/literal/simple binary forms)
• exception filters currently behave as enable/disable (non-empty => enabled, empty => disabled)