ERR/E3: error-tag {} interpolation via an always-linked tag-name table

`{}` on an error-set value printed `<?>` (any_to_string had no error_set
category). Now it renders the tag name (`BadDigit`), reusing the existing
any_to_string dispatch.

Pieces:
- New `error_tag_name_get` IR op (UnaryOp): tag id -> name. Lowered from a new
  `error_tag_name(e) -> string #builtin` (std.sx). Handled across inst.zig
  (op def), print.zig, interp.zig (comptime: tags.getName), and emit_llvm.zig.
- emit_llvm getOrBuildTagNameArray: an always-linked `[N x {ptr,i64}]` global
  of tag names indexed by global tag id (the TagRegistry namespace, slot 0 =
  ""). error_tag_name_get zext's the u32 tag value and GEPs into it. Built once;
  not trace-gated, so it works in release too (per the spec's "tag-name table
  always shipped").
- resolveTypeCategoryTags gains an `error_set` category so the
  `case error_set:` arm in any_to_string matches; that arm coerces the Any to
  u32 (`xx val`) and calls error_tag_name. (cast(type) didn't recover the tag
  id for error-set values; the u32 coercion does.)

examples/240-error-tag-interpolation.sx: bound tags + a catch-bound tag print
their names. Regenerated ffi-objc-call-06-sret-return.ir — pure block-renumber
drift from adding one if-arm to the shared any_to_string (verified
semantically identical after collapsing block numbers).

Gates: zig build, zig build test, bash tests/run_examples.sh (277 passed; lone
failure is the user's uncommitted 213-canonical-map pack WIP).

src/ir/interp.zig

@@ -1227,6 +1227,14 @@ pub const Interpreter = struct {
                 const name = self.module.types.getString(fields[idx].name);
                 return .{ .value = .{ .string = name } };
             },
+            .error_tag_name_get => |u| {
+                const tag_val = frame.getRef(u.operand);
+                const id: u32 = @intCast(switch (tag_val) {
+                    .int => |i| i,
+                    else => return bailDetail("comptime error_tag_name(e): operand is not an integer tag id"),
+                });
+                return .{ .value = .{ .string = self.module.types.tags.getName(id) } };
+            },
             .field_value_get => |fr| {
                 const base_val = frame.getRef(fr.base);
                 const idx_val = frame.getRef(fr.index);