reanalyze: localise dead-code binding and reporting state

[cristianoc]

• Introduce BindingContext in DeadValue to track the current binding location during dead-code traversal, so binding context is explicit and locally encapsulated.
• Introduce ReportingContext in DeadCommon to track, per file, the end position of the last reported value when deciding whether to suppress nested warnings.
• Replace addValueReference_state with addValueReference ~binding, so value-reference bookkeeping is driven by an explicit binding location rather than a threaded analysis state.
• Update dead-code value and exception handling to use the new addValueReference API.
• Refresh DEADCODE_REFACTOR_PLAN.md to mark these state-localisation steps as completed and to narrow the remaining follow-up to making the binding context fully pure.
• Verified with make test-analysis that behaviour and expected outputs remain unchanged.