fix(0126): array args bind []$T generic params; uninferrable type params diagnose at the call

extractTypeParam's slice arm only extracted from slice-typed args, so
first(a) with a : [3]s64 at first :: (xs: []$T) -> T left T unbound
and the mono body reached LLVM emission carrying the .unresolved
sentinel (panic). The arm now also extracts from array args via the
array's element type — mirroring the array→slice promotion concrete
slice params already perform; the existing arg coercion handles the
rest.

lowerGenericCall additionally diagnoses any still-uninferrable TYPE
param at the call site instead of monomorphizing unbound — the
deliberate string-at-[]$T gap used to hit the same sentinel panic and
now errors with a source-located message. Comptime value params
($N: u32) and ..$Ts packs bind through their own dispatch and stay
exempt.

Regressions: examples/0212-generics-array-arg-slice-param.sx (scalar /
u8 / struct elements + the slice spelling) and
examples/1168-diagnostics-generic-param-uninferrable.sx (string arg
diagnostic) — both failed pre-fix.

src/ir/lower/generic.zig

@@ -554,11 +554,15 @@ pub fn extractTypeParam(self: *Lowering, type_node: *const Node, arg_ty: TypeId,
         .type_expr => |te| if (std.mem.eql(u8, te.name, tp_name)) arg_ty else null,
         .identifier => |id| if (std.mem.eql(u8, id.name, tp_name)) arg_ty else null,
         .slice_type_expr => |st| blk: {
-            // arg_ty should be a slice → extract element type
+            // arg_ty should be a slice → extract element type. An array
+            // arg coerces to a slice at a `[]T` param (the same promotion
+            // concrete slice params perform), so it binds from its
+            // element type too (issue 0126).
             if (arg_ty.isBuiltin()) break :blk null;
             const info = self.module.types.get(arg_ty);
             break :blk switch (info) {
                 .slice => |s| self.extractTypeParam(st.element_type, s.element, tp_name),
+                .array => |a| self.extractTypeParam(st.element_type, a.element, tp_name),
                 else => null,
             };
         },