Add module visibility syntax spike

[miselin]

What changed

• Replace the declaration-level public boolean with file, module, and external visibility tiers.
• Add pub(module) syntax alongside existing pub syntax.
• Add visibility blocks such as pub(module) { ... }, desugared during CST-to-surface lowering.
• Preserve visibility for functions, globals, and types, including LLVM linkage behavior and LSP display details.
• Add parser/pipeline coverage and update the grammar and language documentation.

Why

This establishes a readable syntax for sharing declarations across files in one module without exporting them at external/linker scope. The block form reduces repetitive modifiers while remaining surface sugar.

Validation

• opam exec -- dune build
• opam exec -- dune runtest
• git diff --check

The standalone Lark example validator was not run successfully because the active Python environment does not have the lark package installed.

Follow-up

Module identity and visibility enforcement across imported files remain the next semantic slice. This PR establishes the syntax and AST representation needed for that work.