comptime enum value params: $o: EnumType binds+resolves variant tag

A comptime value param whose constraint is a plain enum ($o: Ord) now
binds its enum-literal argument to the variant tag during inlined
comptime-call lowering. The tag is recorded in comptime_value_bindings
(readable downstream via comptimeIntNamed / direct map lookup, and as an
array-dim style const-int leaf) AND the param is bound into scope as an
enum_init value so body comparisons like 'if o == .a' lower as ordinary
enum comparisons. Distinct ordering args monomorphize the inlined body
per value.

A non-constant argument or an unknown variant emits a loud diagnostic
and binds nothing — never a silent default.

Locked by examples/0627-comptime-enum-value-param.sx.

src/ir/lower.zig

@@ -1572,6 +1572,7 @@ pub const Lowering = struct {
     pub const lowerComptimeGlobal = lower_comptime.lowerComptimeGlobal;
     pub const lowerComptimeSideEffect = lower_comptime.lowerComptimeSideEffect;
     pub const lowerComptimeCall = lower_comptime.lowerComptimeCall;
+    pub const bindEnumValueParams = lower_comptime.bindEnumValueParams;
     pub const lowerInlineComptime = lower_comptime.lowerInlineComptime;
     pub const lowerInsertExpr = lower_comptime.lowerInsertExpr;
     pub const lowerInsertExprValue = lower_comptime.lowerInsertExprValue;