Support source-level covenant debugger sessions

[Manyfestation]

Summary

• add source-level covenant declaration metadata to the compiler
• let the debugger CLI launch covenant sessions by source function name, including delegate mode and explicit state fixtures
• expose source-level prev_state and prev_states bindings inside debugger sessions

Testing

• cargo test -p cli-debugger --tests
• cargo test -p debugger-session --tests
• cargo test -p silverscript-lang --test compiler_tests