lang 2.4: lock protocol-pack access semantics (interface-only)

Design decision: a protocol-constrained pack element is viewed THROUGH the
constraint protocol — only the protocol's interface (its methods, and the
projections xs.T / xs.value) is accessible, not arbitrary concrete members,
exactly like a constrained generic `T: Show`. So `xs[i].v` (a field on the
concrete IntBox, not declared on Show) is an error; the constraint is enforced
and bounds the body regardless of the concrete arg types at a call site.

The previous example 191 demonstrated `xs[i].v` — which only compiled because
the constraint is not yet enforced. Trimmed it to the protocol-agnostic part
that's correct today (per-shape binding + comptime `xs.len` across arities /
heterogeneous shapes); protocol-interface access + projection are the remaining
2.4 work. specs.md records the access rule.

tests/expected/191-protocol-pack.txt

@@ -1,4 +1,3 @@
-count0=0
-count2=2
-sum=49
-describe len=2 first=42 second=cool
+n0=0
+n2=2
+n3=3