Easy complexity cleanup: fix diagnostic example scanner stale index

[cuzzo]

Report-backed easy refactor.

Source: Decomplex report-tools-diagnostic-scan / Derived-State Staleness.

Problem:
src/ast/diagnostic_examples.rb#scan_file derives j from i, then continues with stateful scanning. The scanner is small, covered by tests, and suitable for a low-risk cleanup.

Acceptance:

• Refactor scanner state so the next describe index is explicit and not stale-derived.
• Add focused coverage for fix-line scanning and malformed annotations.
• No new T.untyped slots.
• Decomplex/SlopCop metrics stay flat or improve.