chore - v0.6 backend compatibility report and migration notes

[dannymeijer]

Area

• Documentation
• Incan Compiler
• Tooling / CLI

Problem statement

Removing the Rust-source backend changes the compiler architecture and may expose differences between supported behavior, accidental behavior, and generated-artifact behavior. Users and contributors need a clear compatibility report instead of discovering those differences through broken builds.

Proposed solution

Publish a backend compatibility report and migration notes for the backend cutover.

The report should classify:

• behavior preserved from the current compiler;
• behavior intentionally changed;
• behavior removed because it was accidental or generated-artifact-specific;
• diagnostics that changed shape or location;
• stdlib/runtime behavior that required migration;
• known unsupported gaps after cutover.

Scope

In scope:

• compatibility report based on the behavior inventory;
• migration notes for affected users and contributors;
• links to cutover, HIR, Body IR, and backend issues;
• clear distinction between supported behavior and accidental compatibility.

Out of scope:

• implementing the replacement backend;
• broad language feature documentation;
• product-specific Hees.ai documentation.

Done when

• The backend cutover has a written compatibility report.
• Migration notes explain what users and contributors need to change.
• Known gaps are tracked as follow-up issues rather than hidden in release prose.