Map should resolve by StableKey, not section ID

[deanleigh]

Problem

When a new group is inserted into source.json (e.g. adding "More Sights" between "Sights" and "Accommodation"), the auto-generated section IDs shift (e.g. preamble-main-content-p1-4 becomes preamble-main-content-p1-5).

The map.json currently resolves mappings using the source field (the section ID), which means inserting a new group breaks existing mappings — the mapping that pointed at Accommodation now points at the new "More Sights" section instead.

Each section already has a StableKey (the group ID from source.json), and map.json already stores a sourceKey field. But the mapping resolution appears to prioritise the positional section ID over the stable key.

Expected Behaviour

• When a sourceKey is present in a mapping, it should be used to resolve the source section, not the source (section ID) field
• The source field should be treated as a display/fallback reference only
• Inserting, removing, or reordering groups in source.json should not break existing mappings

Reproduction Steps

1. Set up a workflow with groups mapped in map.json (e.g. Sights → block, Accommodation → block)
2. Insert a new group between them in source.json
3. Re-run the source extraction
4. Observe that the mapping for the section after the insertion now points to the wrong section

Real-World Example (Tailored Travel)

• Accommodation was mapped via preamble-main-content-p1-4
• After adding "More Sights" group, Accommodation became preamble-main-content-p1-5
• The Accommodation mapping landed on "More Sights" instead
• Had to manually fix map.json to update both the source ID and the sourceKey

Additional Context

The sourceKey in the existing Accommodation mapping (7df15acf-5938-45ce-a789-9ea2e8483437) did not match the Accommodation group's actual ID (96c2347e-4944-4d26-8a90-757cc45aae18), suggesting the sourceKey may not have been populated correctly when the mapping was originally created via the UI.

Planning

See planning/STABLEKEY_MAP_RESOLUTION.md for investigation notes and implementation approach.