ERR/E1.1 (slice 2): error.X value lowering + enum-like == typing

Completes E1.1. All in ir/lower.zig (the IR layer, per slice 1's finding).

- lowerFieldAccess intercepts `error.X` (parsed as field_access(identifier
  "error", X)) → lowerErrorTagLiteral: interns the tag; when target_type is a
  named error set, types the value as that set and validates X ∈ set (out-of-set
  → diagnostic); otherwise emits the raw u32 global tag id (the spec's
  context-free default — not a silent guess).
- tryLowerErrorSetEquality (early branch in lowerBinaryOp) + errorSetTypeOf /
  isErrorTagLiteralNode: an error-set value or `error.X` literal forces the other
  operand to be one too, else a diagnostic ("compares only with an error.X tag or
  another error-set value; coerce with `xx`"). Both sides lower under the set type
  as context (error.X resolves + membership-checks); two bare tag literals with no
  context compare as global u32 ids. Handles both operand orders.

First ERR examples (end-to-end): 217-error-sets.sx (declared set + error.X +
== true/false + u32 coercion → "error-set result: 25", exit 25) and
218-error-set-typing.sx (out-of-set literal + tag-vs-raw-int → 2 diagnostics).

Failable `!`/`!Named` signatures and raise/try/catch/onfail semantics remain
(E1.2+). zig build, zig build test, and 256/256 examples green.

examples/218-error-set-typing.sx

@@ -0,0 +1,16 @@
+// Error-set value + `==` typing rejections (ERR step E1.1):
+//   - an `error.X` literal must name a tag that is in the destination set,
+//   - an error-set value compares only with an `error.X` tag or another
+//     error-set value; comparing to a raw integer is a type error
+//     (coerce with `xx` to compare the raw id).
+// The positive cases live in `examples/217-error-sets.sx`.
+
+#import "modules/std.sx";
+
+ParseErr :: error { BadDigit, Overflow }
+
+main :: () -> s32 {
+    c : ParseErr = error.NotInSet;   // error: NotInSet not in ParseErr
+    if c == 42 { return 1; }         // error: error-set value vs raw integer
+    return 0;
+}