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/1061-errors-void-failable-fallthrough.sx

@@ -0,0 +1,21 @@
+// A pure-failable function (`-> !`) that succeeds by IMPLICIT fall-through —
+// no explicit `return;` — must initialize its error-channel slot to 0 ("no
+// error"), exactly like an explicit `return;` would. Otherwise the slot is
+// left undefined and a caller's `catch` (or `main`) reads a garbage tag and
+// reports a phantom unhandled error.
+//
+// This exercises:
+//   - a `-> !` callee that falls off the end (no `return;`) — its `catch`
+//     handler must NOT fire;
+//   - a `main :: () -> !` that falls off the end — must exit 0.
+//
+// Regression (issue 0190).
+
+#import "modules/std.sx";
+
+noop :: () -> ! { }                 // success by fall-through, no `return;`
+
+main :: () -> ! {
+    noop() catch (e) { print("phantom: {}\n", e); };   // must NOT fire
+    print("ok\n");                                     // main falls through → exit 0
+}