Move the DWARF debug-info emission out of emit_llvm.zig into a DebugInfo backend
*LLVMEmitter facade (field `e`). Behavior-preserving relocation — self.* ->
self.e.* only.
- src/backend/llvm/debug.zig (DebugInfo): debugEnabled + diFileFor (private) +
initDebugInfo / beginFunctionDebug / endFunctionDebug / setInstDebugLocation /
finalizeDebugInfo (pub). The mutable DI state (di_builder/di_cu/di_files/
di_scope/current_func_file) + the shared source map (import_sources/main_file)
stay on LLVMEmitter; the facade reads/writes them via self.e.*.
- Routed the 5 pass-order call sites in LLVMEmitter.emit (init/finalize/
begin/end/setInstDebugLocation) through a new debugInfo() accessor.
- setDebugContext stays on LLVMEmitter (shared-state setter; callers in main.zig/
core.zig/test). sourceForFile stays on LLVMEmitter and is widened to pub — it is
shared with reflection's trace-frame emission (emitTraceFrame), not debug-only.
- No DI logic / module-flag / DWARF-version / scope-line change.
Gate: zig build, zig build test, bash tests/run_examples.sh -> 361/0 (no churn).
