feat(lang): #76 namespace block form with export/internal visibility

`namespace Name { export function foo(...) ... internal function bar(...) ... }`
declares a Name.* namespace once and controls its public surface declaratively,
instead of annotating every function with @Namespace(Name). Inside the block
each `function short(...)` is emitted as `namelower_short`; export (the default)
makes it callable as Name.short(...), internal keeps it a private helper
(emitted, callable by short name from siblings — calls are rewritten — but
Name.internalOne() is a compile error). Block sugar for the per-function
@Namespace annotation; a package's public API reads at a glance. Namespaces
declared the old way are unchanged (gameplay_foundation still passes).

namespace added to LUDIC_KW_DECL + JetBrains/TextMate + docs page + inventory
(vocab/impl/docs checks green). Example namespace_block (export + internal +
sibling calls + internal-visibility compile error verified). Full suite 116/0,
fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/emit_call.ludic

@@ -689,7 +689,12 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
   # #62: a package-provided namespace (declared with @Namespace(Foo)) that none
   # of the hardcoded core blocks matched — alias Foo.method to the bare function
   # foo_method (positional args), the same generic path the core aliases use.
-  if (bare == null) and is_registered_namespace(ns) { bare = ns_lower(ns) + ("_") + meth }
+  if (bare == null) and is_registered_namespace(ns) {
+    bare = ns_lower(ns) + ("_") + meth
+    # #76: a namespace declared with a block controls its public surface — an
+    # `internal` method is emitted but not callable as Name.method.
+    if is_ns_block(ns) and (not ns_export_has(bare)) { perr(`{ns}.{meth} is internal to namespace {ns}`) }
+  }
   if (bare == null) { perr(`unknown builtin {ns}.{meth}`) }
   reorder_named(e, labels)
   let id = node(E_ID); id.s = bare; e.a = id