ERR/E3.0 (slice 1): thread source spans into IR instructions

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.

src/ir/lower.zig

@@ -1544,6 +1544,11 @@ pub const Lowering = struct {
     }
 
     fn lowerStmt(self: *Lowering, node: *const Node) void {
+        // Stamp this statement's span onto its instructions (ERR E3.0); see
+        // `lowerExpr`.
+        const saved_span = self.builder.current_span;
+        defer self.builder.current_span = saved_span;
+        if (node.span.start != 0 or node.span.end != 0) self.builder.current_span = .{ .start = node.span.start, .end = node.span.end };
         switch (node.data) {
             .var_decl => |vd| self.lowerVarDecl(&vd),
             .const_decl => |cd| self.lowerConstDecl(&cd),
@@ -2253,6 +2258,14 @@ pub const Lowering = struct {
     // ── Expression lowering ─────────────────────────────────────────
 
     fn lowerExpr(self: *Lowering, node: *const Node) Ref {
+        // Stamp this node's source span onto the instructions it emits (ERR
+        // E3.0 — feeds DWARF line-info + comptime frame resolution). Save/
+        // restore so a parent's later emits keep the parent's span after a
+        // child lowers. Skip the empty default so synthetic nodes don't reset
+        // a meaningful enclosing span to offset 0.
+        const saved_span = self.builder.current_span;
+        defer self.builder.current_span = saved_span;
+        if (node.span.start != 0 or node.span.end != 0) self.builder.current_span = .{ .start = node.span.start, .end = node.span.end };
         return switch (node.data) {
             // Bare `$<pack>` in expression position → an `[]Type` slice
             // value where each element is a `const_type(arg_types[i])`.