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.

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

@@ -0,0 +1,27 @@
+// Comptime ENUM value parameter: `$o: <EnumType>` binds the enum-literal
+// argument to its variant tag, monomorphizes the inlined body per distinct
+// ordering value, and resolves `o` in the body as a compile-time-known enum
+// constant — usable both in `if o == .a` comparisons AND as a comptime-readable
+// variant tag during lowering (a downstream lowerer reads it via
+// `comptime_value_bindings`, exercised here as the `[o]i64` array dimension).
+#import "modules/std.sx";
+
+Ord :: enum { a; b; c; }
+
+pick :: ($o: Ord) -> i64 {
+    if o == .a { return 10; }
+    if o == .b { return 20; }
+    return 30;
+}
+
+// `o` read as a compile-time integer (its variant tag) in a TYPE position.
+tag_dim :: ($o: Ord) -> i64 {
+    arr : [o]i64 = ---;
+    return arr.len;
+}
+
+main :: () {
+    print("{}\n", pick(.b));                       // 20
+    print("{} {} {}\n", pick(.a), pick(.b), pick(.c)); // 10 20 30
+    print("{} {} {}\n", tag_dim(.a), tag_dim(.b), tag_dim(.c)); // 0 1 2
+}

examples/expected/0627-comptime-enum-value-param.exit

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

examples/expected/0627-comptime-enum-value-param.stderr

@@ -0,0 +1 @@
+

examples/expected/0627-comptime-enum-value-param.stdout

@@ -0,0 +1,3 @@
+20
+10 20 30
+0 1 2