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/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 @@
+