fix: emit mem.create and mem.alloc_slice on the M7 path

m7_emit_call reimplements call emission and only carried over mem.destroy and
mem.replace, so every mem.create/mem.alloc_slice fell through to the generic
member path and emitted `fe_missing.create(0)`. wcc386 does not diagnose that
-- it terminates with exit 255, which wcl386 reports as "Unable to invoke
wcc386.exe" with no message at all.

The breakage covered all 17 allocation sites in m5/runtime.fe, and it was
invisible because m5-owned and m5-defer only emit C; m5-runtime is the one M5
fixture that compiles and links what was generated.

Also make runtime.fe legal: `run` used `try` while returning i32, which SPEC
allows only in a function returning an error union. It is `-> !i32` now, which
M7 accepts because contextual success construction lands with it, and
runtime.c takes the { error, value } struct the error union lowers to.

Found by calling wcc386 directly instead of through wcl386, which is the only
way to see a compiler crash here.

M1-M7: 183 passed.

fec/tests/m5/runtime.c

@@ -7,7 +7,9 @@
 extern void *malloc(size_t size);
 extern void free(void *p);
 
-extern long fe_m5_runtime_run(long mode);
+/* `run` returns !i32, which lowers to { error, value }. */
+struct fe_result_value_9 { unsigned short e; long v; };
+extern struct fe_result_value_9 fe_m5_runtime_run(long mode);
 extern unsigned short fe_m5_runtime_conditional(unsigned char flag);
 extern unsigned short fe_m5_runtime_argument_cleanup(void);
 extern unsigned short fe_m5_runtime_owned_slice(unsigned long n);
@@ -64,9 +66,10 @@ void m5_free(void *p)
 
 int main(void)
 {
-    if (fe_m5_runtime_run(0) != 0) return 1;
-    if (fe_m5_runtime_run(1) != 9) return 2;
-    if (fe_m5_runtime_run(2) != 0) return 3;
+    struct fe_result_value_9 r;
+    r = fe_m5_runtime_run(0); if (r.e != 0 || r.v != 0) return 1;
+    r = fe_m5_runtime_run(1); if (r.e != 0 || r.v != 9) return 2;
+    r = fe_m5_runtime_run(2); if (r.e != 0 || r.v != 0) return 3;
     if (fe_m5_runtime_conditional(0) != 0) return 4;
     if (fe_m5_runtime_conditional(1) != 0) return 5;
     if (fe_m5_runtime_argument_cleanup() != 0) return 6;

fec/tests/m5/runtime.fe

@@ -2,7 +2,7 @@ unit m5_runtime;
 
 fn take(p: ^i32) -> void { mem.destroy(p); }
 
-pub fn run(mode: i32) -> i32 {
+pub fn run(mode: i32) -> !i32 {
     var p: ^i32 = try mem.create(0);
     defer { mem.destroy(p); }
     p.^ = 7;