feat(t27b): plain enums as integer tags (Closes #6146)

[gHashTag]

Closes #6146. Stacked on #6249 (slices), which is stacked on #6247 (arrays).

What

• enum E { .. } and const E = enum(T) { .. }, with and without explicit values. The tag type is mirrored from t27c's Zig backend:
  • enum(T) uses T;
  • no tag type but some explicit value uses i32;
  • otherwise the smallest unsigned type, as Zig picks it.
• Enum values: E.v, E::v, and .v wherever an E is expected (args, returns, typed locals, struct fields, module constants, assert_eq, @as(E, .v)).
• == and != compare tags. Ordering is accepted only where t27c writes @intFromEnum(a) < @intFromEnum(b) (one side E.v).
• @intFromEnum, plus @as(T, x) in general.
• @enumFromInt:
  • At runtime it range-checks and traps invalid enum value (TrapKind::EnumTag), as Zig's safety check does.
  • A constant that is no tag is rejected (a Zig compile error).
• Clean rejects, each with its own construct:
  • EnumDecl(method), EnumDecl(non-exhaustive), EnumDecl(union) (for union(enum));
  • ExprSwitch (t27b has no switch yet);
  • ExprBinary(<) on enum for two enum values;
  • ExprEnumValue for .v with no result type;
  • a non-contiguous runtime @enumFromInt;
  • a runtime @intFromEnum of an auto-tagged (u2) enum used as a value (allowed in comparisons and where a wider integer is wanted).
• No interpreter fallback and no skipped tests. The new run test asserts JIT == interpreter, including both EnumTag trap sites.

Corpus (t27b corpus specs, macOS arm64)

	before	after
pass	162 (938 tests)	165 (973 tests)
pass_vacuous	278	278
fail	2	2 (same files: build_verify.t27, viewport.t27)
rejected	763	760
JIT/interpreter mismatch	0	0

• EnumDecl was the 📜 Canon Update: AGENTS_ALPHABET + AGENT_T_SKILL #1 first blocker (109 files) and is now gone from the list.
• New top first blockers: BenchBlock 116, type f64 107, type f32 69, type ?T 47, std.mem.Allocator 44, type [N]T 39.
• New passes, with the reference result:
  • AsmSectionKind_from_i8.t27: reference 8/8.
  • HirBusPort_axi4_lite_slave.t27: reference 23/23.
  • server/api.t27: the reference does not compile it, because an unused struct names an undeclared type. Zig catches that at AstGen; t27b lays out structs lazily, which predates this PR.
• No new FAILs.

Reference (t27c Zig backend) bugs found

• const D: Dir = .east; is emitted as const D: Dir;, so the value is dropped and the file fails with "variables must be initialized".
• In a test block, var x: u8 = 1; x = f(x); is emitted with const x = f(x);, so zig fails with "redeclaration of local variable".

🤖 Generated with Claude Code