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).

library/modules/std.sx

@@ -22,6 +22,7 @@ field_value :: (s: $T, idx: s64) -> Any #builtin;
 is_flags :: ($T: Type) -> bool #builtin;
 field_value_int :: ($T: Type, idx: s64) -> s64 #builtin;
 field_index :: ($T: Type, val: T) -> s64 #builtin;
+error_tag_name :: (e: $T) -> string #builtin;
 string :: []u8 #builtin;
 
 #import "allocators.sx";
@@ -314,6 +315,7 @@ any_to_string :: (val: Any) -> string {
         case float: result = float_to_string(xx val);
         case struct: result = struct_to_string(cast(type) val);
         case enum: result = enum_to_string(cast(type) val);
+        case error_set: { tagid : u32 = xx val; result = error_tag_name(tagid); }
         case vector: result = vector_to_string(cast(type) val);
         case array: result = array_to_string(cast(type) val);
         case slice: result = slice_to_string(cast(type) val);