ERR/E1.1 (slice 1): error-set type + global tag registry + decl registration

First sema/types step. Implemented in the IR layer (ir/types.zig +
type_bridge.zig + lower.zig), NOT src/sema.zig — lowering doesn't consume
sema; the frontend Type is LSP-only. Mirrors how enums are handled.

- ir/types.zig: new `.error_set` TypeInfo kind (ErrorSetInfo {name, tags:
  []u32}; identity = name, like enum) with a u32 runtime layout (size/align
  4, LLVM i32) per the locked error-slot ABI. New TagRegistry on TypeTable
  (global tag pool: name -> u32, monotonic, id 0 reserved for "no error").
  internTag/getTagName/errorSetType helpers; `.error_set` arms in all 7
  exhaustive switches + findByName.
- emit_llvm: toLLVMTypeInfo -> i32. print: writeType -> set name.
- type_bridge: resolveInlineErrorSet (mirrors resolveInlineUnion) +
  .error_set_decl arm.
- lower.zig: registerErrorSetDecl (rejects empty `error { }` with a
  diagnostic) wired into both top-level decl switches + the block-local one.
- tests: ir/types.test (TagRegistry 0-reserved + identity; errorSetType u32
  layout + named display + dedup; sorted storage) and ir/type_bridge.test
  (decl -> type + tag interning + re-resolve dedup).

End-to-end: `Foo :: error { A, B }` + main compiles + runs (exit 0) — first
ERR syntax to survive the full pipeline; empty set rejects with a diagnostic.
Inferred bare `!`, error.X value, and == typing deferred to slice 2 / E1.2.

zig build, zig build test, and 254/254 examples green.

src/ir/emit_llvm.zig

@@ -4182,6 +4182,7 @@ pub const LLVMEmitter = struct {
             .f64 => self.cached_f64,
             .void => self.cached_void,
             .bool => self.cached_i1,
+            .error_set => self.cached_i32, // u32 tag id on the error channel
             .string => self.getStringStructType(),
             .pointer, .many_pointer, .function => self.cached_ptr,
             .closure => self.getClosureStructType(),