issue 0131: protocol method calls silently drop extra arguments

issues/0131-protocol-call-extra-args-silently-dropped.md

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+# 0131 — protocol method call with extra arguments compiles and silently drops them
+
+## Symptom
+
+Calling a protocol method with MORE arguments than the protocol
+declares is accepted by the compiler; the extra arguments are silently
+dropped at the call. Observed: `Allocator.dealloc_bytes` is declared
+`(ptr: *void)`, yet `a.dealloc_bytes(p, 12345)` compiles and runs
+under both `sx run` and `sx build`. Expected: a compile diagnostic
+("expected 1 argument, got 2"), exactly as for plain function calls.
+
+Found via std.http (PLAN-HTTPZ S7a): three `dealloc_bytes(ptr, size)`
+calls — written against an imagined two-arg signature — compiled
+clean and survived the full example sweep. Corrected in `81fa50c`;
+this issue is about the missing diagnostic.
+
+## Reproduction
+
+```sx
+#import "modules/std.sx";
+
+main :: () -> i32 {
+    gpa := GPA.init();
+    a : Allocator = xx gpa;
+    p := a.alloc_bytes(64);
+    a.dealloc_bytes(p, 12345);   // protocol declares (ptr) — must be rejected
+    print("compiled and ran\n");
+    return 0;
+}
+```
+
+Run `sx run repro.sx`: prints "compiled and ran", exit 0. Expected: a
+compile error naming the arity mismatch.
+
+## Investigation prompt
+
+The sx compiler accepts protocol method calls with extra trailing
+arguments and silently drops them (repro above — `Allocator.
+dealloc_bytes(ptr)` called with `(ptr, extra)` compiles and runs).
+Plain function calls DO arity-check, so the gap is specific to the
+protocol-dispatch call path. Suspected area: the protocol call
+planning/lowering in `src/ir/lower/call.zig` (and/or the protocol
+method resolution in `src/ir/protocols.zig`) — wherever a protocol
+method's parameter list is matched against call-site args, the
+arg-count check that plain calls get is likely skipped, and lowering
+then truncates args to the method's param count. The fix should emit
+the same diagnostic plain calls produce (expected N args, got M) for
+both too many AND verify too few is also caught. Verify with the
+repro (expect a compile error) plus a negative-test example or a
+`call.test.zig` case pinning the diagnostic; then `zig build &&
+zig build test && bash tests/run_examples.sh` all green (existing
+examples must not regress — if any example relied on dropped extra
+args, that example is itself a latent bug to fix in the same
+session).