Add global imperative variables with heap-aware debugger scopes

[ejgallego]

Summary:

• extend ImpLab language with top-level global declarations and get/set operations
• model runtime heap as a persistent map in Context, with runtime errors for undeclared globals
• expose locals + heap scopes through debugger core, DAP, and widget views
• update examples and documentation to teach the new global heap model
• add regression coverage for heap persistence across calls and heap visibility in callee frames

Validation:

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

Notes:

• heap scope variables use dedicated references in core (locals odd refs, heap even refs).