Problem

There are no defined compatibility guarantees between NLS versions. Users cannot know if upgrading the toolchain will break their .nl files.

Acceptance Criteria

• Defined semver policy: what constitutes a breaking change to the language spec
• @nls_version directive or equivalent in .nl files
• Compiler warns or errors on version mismatches
• CHANGELOG tracks spec-level changes separately from toolchain changes

Why This Blocks v0.5

A language without version semantics can't be adopted for real projects.