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/module.zig

@@ -253,6 +253,12 @@ pub const Builder = struct {
     current_block: ?BlockId = null,
     /// Running instruction counter within the current function (for Ref assignment).
     inst_counter: u32 = 0,
+    /// Source span stamped onto every instruction emitted via `emit`/`emitVoid`
+    /// (ERR E3.0). Lowering sets it (save/restore) at each AST node so the IR
+    /// carries per-instruction locations for DWARF `.debug_line` + comptime
+    /// frame resolution. Defaults empty for instructions emitted outside a
+    /// node context (synthetic prologue/epilogue, etc.).
+    current_span: Span = .{},
 
     pub fn init(module: *Module) Builder {
         return .{ .module = module };
@@ -344,7 +350,7 @@ pub const Builder = struct {
     // ── Emit helpers ────────────────────────────────────────────────
 
     pub fn emit(self: *Builder, op: Op, ty: TypeId) Ref {
-        return self.emitSpan(op, ty, .{});
+        return self.emitSpan(op, ty, self.current_span);
     }
 
     fn emitSpan(self: *Builder, op: Op, ty: TypeId, span: Span) Ref {
@@ -359,7 +365,7 @@ pub const Builder = struct {
     pub fn emitVoid(self: *Builder, op: Op, ty: TypeId) void {
         const block = self.currentBlock();
         self.inst_counter += 1;
-        block.insts.append(self.module.alloc, .{ .op = op, .ty = ty }) catch unreachable;
+        block.insts.append(self.module.alloc, .{ .op = op, .ty = ty, .span = self.current_span }) catch unreachable;
     }
 
     // ── Constants ───────────────────────────────────────────────────