fix: initialize the error-channel slot on every failable implicit success return (issue 0190)

A failable function that returned by IMPLICIT success (no explicit
`return`) left its error-tag slot uninitialized, so a caller's `catch` /
`or` (or `main`) read a garbage tag and reported a phantom unhandled
error — and for value-carrying failables the success value was dropped.
The "no error" sentinel was only written on the explicit-`return;` path.

Unified all function-body-return lowering so the failable-success slot
is always written:
  - void `-> !` fall-through: `ensureTerminator` (control_flow.zig) now
    emits `ret constInt(0)` for a pure-failable end-of-body.
  - value-failable trailing-expression success: `lowerValueBody`
    (stmt.zig) routes through `lowerFailableSuccessReturn`.
  - generic + pack-fn instances: `monomorphizeFunction` (generic.zig) and
    `monomorphizePackFn` (pack.zig) now DELEGATE their body-return to
    `lowerValueBody` instead of hand-rolling a `coerce`+`ret` that drifted
    (covers generic/pack value-failables).

Also fixes the missing-value diagnostic guard added here: it now counts
`.err`-level diagnostics (new `DiagnosticList.errorCount`) rather than the
total list length, so a warning/note emitted while lowering the body
(e.g. an ObjC selector arity warning) can no longer suppress a genuine
"body produces no value" error — which previously shipped an
uninitialized return at exit 0.

Regressions: examples/errors/1061 (void fall-through), 1062 (value-failable
trailing expr), 1063 (generic value-failable trailing expr).

examples/errors/1062-errors-value-failable-trailing-expr.sx

@@ -0,0 +1,43 @@
+// Value-carrying failable functions (`-> T !E`) whose body ends in a trailing
+// success EXPRESSION (no explicit `return`) must set the success error slot to
+// 0 — the caller's `catch` must NOT fire and the success value must be intact.
+// Regression (issue 0190): `lowerValueBody` used to `coerceToType`+`ret` the
+// bare success value to the full failable tuple, leaving the error-tag slot
+// uninitialized → phantom catch on success (and dropped value for string /
+// multi-value returns).
+#import "modules/std.sx";
+
+E :: error { Bad }
+
+// Single-value trailing-expression success.
+val :: () -> i64 !E { 99 }
+
+// String trailing-expression success.
+sval :: () -> string !E { "hi" }
+
+// Multi-value (tuple) trailing-expression success.
+mval :: () -> Tuple(i64, i64) !E { .(1, 2) }
+
+// A real error still propagates through the value-failable channel.
+fval :: (n: i64) -> i64 !E {
+    if n < 0 { raise error.Bad; }
+    n + 1
+}
+
+main :: () -> i32 {
+    x := val() catch (e) { print("PHANTOM val\n"); return 1; };
+    print("x={}\n", x);
+
+    s := sval() catch (e) { print("PHANTOM sval\n"); return 1; };
+    print("s={}\n", s);
+
+    t := mval() catch (e) { print("PHANTOM mval\n"); return 1; };
+    print("t=({},{})\n", t.0, t.1);
+
+    ok := fval(10) catch (e) { print("PHANTOM fval-ok\n"); return 1; };
+    print("ok={}\n", ok);
+
+    fval(-1) catch (e) { print("real error caught\n"); return 0; };
+    print("UNEXPECTED no error\n");
+    return 1;
+}