lang F1 2.7: pack-as-value diagnostics (Phase 2 complete)

Using a bare pack name where a runtime value is required was silent garbage
(f(xs)/return xs produced a stray pointer). Now a clear, context-tailored
compile error: isPackName + diagPackAsValue, caught at lowerVarDecl (storage),
lowerReturn (return), lowerFor (iterate), and an identifier-arm catch-all for
call/other. Storage binds a placeholder so there is no cascade error.

Suggestions point at WORKING fixes -- materialize (..xs), or declare the slice
form ..xs: []P for runtime use. The plan category-B "spread ..xs" is broken
(spreading a comptime pack into a []Any param crashes the LLVM verifier; filed
issue 0053), so the diagnostics steer to the slice-of-protocol variadic instead.

Repurposed examples/162-pack-bare-args.sx (was an aspirational bare-$args->[]Any
auto-materialise, contradicting Decision 1) into the slice-form forward
(..args: []Any). examples/203 is the four-category negative test. specs.md "Pack
as value" updated. 238 examples + unit green.

examples/203-pack-as-value.sx

@@ -0,0 +1,25 @@
+// Step 2.7 — pack-as-value diagnostics. A pack is comptime-only (Decision 1),
+// so using the bare pack name where a runtime value is required is an error,
+// with a context-tailored suggestion. All four categories below fire (the
+// functions are monomorphized when called from main).
+
+#import "modules/std.sx";
+
+Show :: protocol { show :: () -> string; }
+A :: struct {}
+impl Show for A { show :: (self: *A) -> string => "A"; }
+
+sink :: (v: s64) -> void { _ = v; }
+
+storage :: (..xs: Show) -> void { y := xs; _ = y; }          // A: store
+call    :: (..xs: Show) -> void { sink(xs); }                // B: pass to a call
+ret     :: (..xs: Show) -> s64  { return xs; }               // C: return
+iter    :: (..xs: Show) -> void { for xs : (x) { _ = x; } }  // D: runtime iterate
+
+main :: () -> s32 {
+    storage(A.{});
+    call(A.{});
+    _ = ret(A.{});
+    iter(A.{});
+    0;
+}