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.

examples/1168-diagnostics-generic-param-uninferrable.sx

@@ -0,0 +1,19 @@
+// A direct call to a generic fn whose arguments cannot bind a TYPE
+// param diagnoses at the call site instead of monomorphizing with the
+// param unbound. A `string` arg at a `[]$T` param is the canonical
+// uninferrable shape (string deliberately does not bind `[]$T`); it
+// used to stamp `.unresolved` through the body and PANIC the compiler
+// at LLVM emission via the sentinel tripwire.
+//
+// Regression (issue 0126, diagnostic half).
+
+#import "modules/std.sx";
+
+first :: (xs: []$T) -> T {
+    return xs[0];
+}
+
+main :: () -> s32 {
+    print("{}\n", first("abc"));
+    return 0;
+}