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/0212-generics-array-arg-slice-param.sx

@@ -0,0 +1,41 @@
+// An ARRAY argument at a generic slice param (`xs: []$T`) binds T from
+// the array's element type — the same array→slice promotion concrete
+// slice params perform — for scalar and struct elements, with the
+// return-position `T` resolving to the element type. The slice
+// spelling keeps working unchanged.
+//
+// Regression (issue 0126): the binding extractor only accepted slice
+// args, so `first(a)` left T unbound and the monomorphized body
+// reached LLVM emission with the `.unresolved` sentinel (panic).
+
+#import "modules/std.sx";
+
+P :: struct { x: s64; y: s64; }
+
+first :: (xs: []$T) -> T {
+    return xs[0];
+}
+
+last :: (xs: []$T) -> T {
+    return xs[xs.len - 1];
+}
+
+main :: () -> s32 {
+    a : [3]s64 = ---;
+    a[0] = 7; a[1] = 8; a[2] = 9;
+    print("{}\n", first(a));
+    print("{}\n", last(a));
+
+    bs : [4]u8 = ---;
+    bs[0] = 5; bs[1] = 6; bs[2] = 7; bs[3] = 8;
+    print("{}\n", last(bs));
+
+    ps : [2]P = ---;
+    ps[0] = .{ x = 1, y = 2 };
+    ps[1] = .{ x = 3, y = 4 };
+    print("{}\n", first(ps).y);
+
+    s : []s64 = a;
+    print("{}\n", first(s));
+    return 0;
+}

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;
+}

examples/expected/0212-generics-array-arg-slice-param.exit

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

examples/expected/0212-generics-array-arg-slice-param.stderr

@@ -0,0 +1 @@
+

examples/expected/0212-generics-array-arg-slice-param.stdout

@@ -0,0 +1,5 @@
+7
+9
+8
+2
+7

examples/expected/1168-diagnostics-generic-param-uninferrable.exit

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

examples/expected/1168-diagnostics-generic-param-uninferrable.stderr

@@ -0,0 +1,5 @@
+error: cannot infer generic type parameter 'T' for 'first' from this call's arguments
+  --> examples/1168-diagnostics-generic-param-uninferrable.sx:17:19
+   |
+17 |     print("{}\n", first("abc"));
+   |                   ^^^^^

examples/expected/1168-diagnostics-generic-param-uninferrable.stdout

@@ -0,0 +1 @@
+