[IDE debugger] Refactor VM values_impl.rs debugging code

[mkurnikov]

Extracted from #19580.

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Note

Medium Risk
Touches VM debugging/trace plumbing and value pretty-printing, which can affect developer tooling and test output; main risk is regressions in debug formatting or thread-local propagation rather than runtime execution semantics.

Overview
Refactors the Move VM debugger by splitting the old monolithic debug.rs into a debug module with separate stdin-command handling and an interpreter-facing InterpreterDebugInterface (now includes get_stack_depth and load_struct_type).

Reworks source/locals/value rendering: source_locator now builds local name metadata, formats locals via a new typed DebugValue serializer (supporting struct/enum field names and special-casing string::String), and exposes get_source_locator(); the old values_impl.rs debug printer is replaced with values_impl/debug.rs plus tests.

Makes debugging state thread-local in tracing (debug enablement + DebugContext) and updates parallel test runners (move-unit-test, transactional runner) to propagate the source locator and debugging flags to Rayon threads and to use source_locator::print_value for output.

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[mkurnikov]

Superseded by #19899