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.

tests/expected/217-error-sets.exit

@@ -0,0 +1 @@
+25

tests/expected/217-error-sets.txt

@@ -0,0 +1 @@
+error-set result: 25

tests/expected/218-error-set-typing.exit

@@ -0,0 +1 @@
+1

tests/expected/218-error-set-typing.txt

@@ -0,0 +1,11 @@
+error: error tag 'error.NotInSet' is not in error set 'ParseErr'
+  --> /Users/agra/projects/sx/examples/218-error-set-typing.sx:13:20
+   |
+13 |     c : ParseErr = error.NotInSet;   // error: NotInSet not in ParseErr
+   |                    ^^^^^^^^^^^^^^
+
+error: an error-set value compares only with an `error.X` tag or another error-set value; coerce with `xx` to compare the raw id
+  --> /Users/agra/projects/sx/examples/218-error-set-typing.sx:14:8
+   |
+14 |     if c == 42 { return 1; }         // error: error-set value vs raw integer
+   |        ^