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/print.zig

@@ -359,6 +359,7 @@ fn printInst(instruction: *const Inst, ref_idx: u32, tt: *const TypeTable, write
         // ── Reflection ──────────────────────────────────────────
         .field_name_get => |fr| try writer.print("field_name_get T{d}[%{d}] : ", .{ fr.struct_type.index(), fr.index.index() }),
         .field_value_get => |fr| try writer.print("field_value_get %{d}, T{d}[%{d}] : ", .{ fr.base.index(), fr.struct_type.index(), fr.index.index() }),
+        .error_tag_name_get => |u| try writer.print("error_tag_name_get %{d} : ", .{u.operand.index()}),
 
         // ── Terminators ─────────────────────────────────────────
         .br => |b| {