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.

tests/expected/203-pack-as-value.exit

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+1

tests/expected/203-pack-as-value.txt

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+error: pack 'xs' has no runtime value — a pack is comptime-only and can't be used as a value here
+  --> /Users/agra/projects/sx/examples/203-pack-as-value.sx:14:40
+   |
+14 | storage :: (..xs: Show) -> void { y := xs; _ = y; }          // A: store
+   |                                        ^^
+
+help: to store it, materialize a tuple: `(..xs)`
+   |
+14 | storage :: (..xs: Show) -> void { y := xs; _ = y; }          // A: store
+   |                                        ^^
+
+error: pack 'xs' has no runtime value — a pack is comptime-only and can't be used as a value here
+  --> /Users/agra/projects/sx/examples/203-pack-as-value.sx:15:40
+   |
+15 | call    :: (..xs: Show) -> void { sink(xs); }                // B: pass to a call
+   |                                        ^^
+
+help: materialize a tuple `(..xs)` to store it, or declare `xs` as a slice variadic `..xs: []P` for runtime use instead of a pack `..xs: P`
+   |
+15 | call    :: (..xs: Show) -> void { sink(xs); }                // B: pass to a call
+   |                                        ^^
+
+error: pack 'xs' has no runtime value — a pack is comptime-only and can't be used as a value here
+  --> /Users/agra/projects/sx/examples/203-pack-as-value.sx:16:42
+   |
+16 | ret     :: (..xs: Show) -> s64  { return xs; }               // C: return
+   |                                          ^^
+
+help: to return it, return a tuple `(..xs)` and make the return type that tuple
+   |
+16 | ret     :: (..xs: Show) -> s64  { return xs; }               // C: return
+   |                                          ^^
+
+error: pack 'xs' has no runtime value — a pack is comptime-only and can't be used as a value here
+  --> /Users/agra/projects/sx/examples/203-pack-as-value.sx:17:39
+   |
+17 | iter    :: (..xs: Show) -> void { for xs : (x) { _ = x; } }  // D: runtime iterate
+   |                                       ^^
+
+help: to iterate at comptime use `inline for 0..xs.len (i)`; for a runtime loop declare it as `..xs: []P` (a protocol slice) instead of a pack
+   |
+17 | iter    :: (..xs: Show) -> void { for xs : (x) { _ = x; } }  // D: runtime iterate
+   |                                       ^^