Parse proc declarations and specification clauses

[jsiek]

Summary

• Add Proc/ProcParam/ProcSpec AST nodes that preserve typed and ghost params, optional returns, and repeated specification clauses.
• Parse proc declarations in both RD and LALR, including requires, labeled/unlabeled ensures, reads, modifies, and decreases clauses with an empty body placeholder.
• Reject proc declarations during checking with a clear parser/AST-only unsupported diagnostic, plus fixture and parser round-trip coverage.

Closes #962

Tests

• make tests

Codex session
codex://threads/019ebf56-ac4d-7e61-8d30-b6fc4873c945

open 'codex://threads/019ebf56-ac4d-7e61-8d30-b6fc4873c945'