fix: protocol method calls arity-check (issue 0131)

emitProtocolDispatch now requires the user-arg count to equal the
protocol method's parameter list — exact, since protocol signatures
have no defaults, packs, or variadics — and emits the same
"expects N arguments, but M were given" diagnostic plain calls get.
Previously extra args were silently dropped (and missing args left the
thunk reading garbage). The dispatch gains the call-site span for the
diagnostic. examples/1634 pins the rejection; full sweep confirms no
existing code relied on the leniency.

examples/1634-protocol-call-arity.sx

@@ -0,0 +1,12 @@
+// issue 0131 regression: a protocol method call must arity-check like a
+// plain call. `Allocator.dealloc_bytes` declares (ptr); calling it with
+// an extra argument used to compile and silently drop the extra.
+#import "modules/std.sx";
+
+main :: () -> i32 {
+    gpa := GPA.init();
+    a : Allocator = xx gpa;
+    p := a.alloc_bytes(64);
+    a.dealloc_bytes(p, 12345);
+    return 0;
+}

examples/expected/1634-protocol-call-arity.exit

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

examples/expected/1634-protocol-call-arity.stderr

@@ -0,0 +1,5 @@
+error: 'dealloc_bytes' expects 1 argument, but 2 were given
+  --> examples/1634-protocol-call-arity.sx:10:5
+   |
+10 |     a.dealloc_bytes(p, 12345);
+   |     ^^^^^^^^^^^^^^^