Foundation for DWARF line-info (E3.0). The `Inst.span` field existed but was
never populated — `emit()` always passed the empty `{0,0}` default, so every
instruction had no source location (the lone reader, the interp's comptime
bail-offset, was always 0).
- Builder gains a `current_span`; `emit`/`emitVoid` stamp it onto each
instruction.
- `lowerExpr` / `lowerStmt` set `current_span` from the AST node's span on
entry and restore it on exit (save/restore), so a parent's later emits keep
the parent's span after a child lowers; the empty default is skipped so
synthetic nodes don't reset a meaningful enclosing span.
Behavior-neutral: codegen never reads spans, and the only consumer (the interp
bail-offset) merely gains real offsets. 290 examples pass unchanged, no `.ir`
snapshot drift. New unit test asserts an emitted `add` carries its `a + b` span.
Next (slice 2): bind `llvm-c/DebugInfo.h`, emit DICompileUnit / DISubprogram /
DIFile / DILocation from these spans, gate on debug/trace mode.
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