Add manual instructions for automatic residual solver codes

[jurei733]

Summary

Relates to #165. This PR intentionally does not close the issue.

• adds a shared default manual instruction for residual fallback codes
• shows manual instructions for RESIDUAL_AUTO only on SOLVER variables
• backfills the default for existing SOLVER residual-auto codes when they are loaded or updated in the editor
• suppresses the generic residual-auto warning for the SOLVER-specific manual-instruction case
• adds focused tests for the default and visibility behavior

Validation

• npm test
• npm run lint (passes with one pre-existing max-len warning in generate-coding-dialog.component.spec.ts)
• npm run build_cc
• git diff --check