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/162-pack-bare-args.sx

@@ -1,25 +1,19 @@
-// Variadic heterogeneous type packs — follow-up #3 (bare `args`
-// reference). The pack-mono doesn't materialise a slice value
-// for the pack name itself, so any body that references `args`
-// without indexing (e.g. forwarding to a `[]Any`-typed helper,
-// or just computing `args.len` through a non-comptime path)
-// fails with "unresolved 'args'".
+// Step 2.7 — forwarding a variadic to a `[]Any` helper.
 //
-// Next commit materialises an `[]Any` slice on demand inside
-// the mono: each pack param is boxed into Any, stored in a
-// stack [N x Any] array, and the slice {data_ptr, len} is bound
-// to the pack name. `args` then resolves as a runtime value
-// like the pre-2b inline path did.
+// A comptime pack `..$args` is comptime-only (Decision 1): `args` bare is NOT a
+// runtime value, so `log_count(args)` on a pack is an error (see the
+// pack-as-value tests). To forward a variadic to a runtime `[]Any` helper,
+// declare it as the *slice* variadic `..args: []Any` — then `args` is a real
+// `[]Any` slice that passes straight through.
 
 #import "modules/std.sx";
 
-// Helper that takes a slice. Today the pack body can't pass
-// `args` here because `args` isn't in scope as a value.
 log_count :: (items: []Any) -> s64 {
     return items.len;
 }
 
-forward :: (..$args) -> s64 {
+// Slice variadic: `args` is a runtime []Any, forwarded directly.
+forward :: (..args: []Any) -> s64 {
     return log_count(args);
 }
 

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;
+}