Title

Promote compiler structured diagnostics to first-class default contract

Problem

Compiler diagnostics are partially structured and partially legacy-message-derived. This leaves language-server and tooling code with fallback heuristics that should become rare.

Why It Matters

• Reduces diagnostic-location drift between compiler and editor integrations.
• Improves reliability for external contributors and downstream tooling.
• Lowers maintenance burden by reducing regex/message parsing paths.

Acceptance Criteria

• Compiler emits structured diagnostics for the primary strict/conformance failure families by default.
• Structured fields include at least: severity, message, and one of (span, lineno, label_id+node_id).
• Existing language-server/tests continue to pass with structured diagnostics preferred over fallback.
• Docs updated where diagnostic contract expectations are described.

Non-Goals

• Redesigning language semantics.
• Removing all fallback logic in one step.
• UI redesign for editor diagnostics.