feat: multi-field object Update syntax

[KrisSimon]

Migrated from GitLab MR !210 (merged)
feature/update-object → main
Originally created: 2026-03-24
Merged: 2026-03-28
Author: Kris Simon

Summary

The `Update` action already supported merging dictionaries at the runtime level. This MR surfaces that capability through the existing `with { }` object-literal syntax — no runtime changes needed.

New syntax:
```aro
Update the with {
modified: <stat-entry: modified>,
fileId: <stat-entry: id>,
filePath: <stat-entry: path>,
fileSize: <stat-entry: size>
} when .
```

This replaces four single-field `Update` statements with one. The `when` guard applies atomically to all fields. Field-access expressions (`<obj: field>`) are valid on the right-hand side.

Changes

• `Proposals/ARO-0004-actions.md`: Added "Update Action — Multi-Field Syntax" section with before/after example; updated action summary description
• `Book/TheLanguageGuide/AppendixA-ActionReference.md`: Added the previously missing `### Update` reference section covering both single-field and multi-field syntax, behaviour notes, and examples
• `Examples/ObjectUpdate/`: New example demonstrating both forms and the `when` guard, with `expected.txt` and `test.hint`

Test plan

• `aro run Examples/ObjectUpdate` matches `expected.txt`
• `aro check Examples/ObjectUpdate` reports no issues
• Single-field update still works unchanged
• Multi-field update with `when ` applies all fields
• Multi-field update with `when ` leaves object unchanged
• Field-access expressions on the right-hand side resolve correctly