Main Street: Grid-based street adjacency (distance-based) #777

Problem statement

Main Street currently stores street cards in a 10-slot array but adjacency/synergy still behaves like a linear strip. The scene now renders a 2x5 board, so adjacency should be distance-based on that grid.

Users

• Players: expect nearby cards on the 2x5 board to synergize consistently with visual layout.
• Designers/QA: need deterministic, testable adjacency that matches board geometry.
• Developers: need reusable mapping helpers from slot index to grid cell and distance-based neighbor calculation.

Success criteria

1. Adjacency uses 2x5 grid coordinates (5 columns x 2 rows) instead of linear neighbors.
2. Default synergy range (range=1) counts orthogonal neighbors by Manhattan distance (up/down/left/right), no diagonals.
3. Extended range from upgrades (synergyRangeBonus) expands Manhattan radius correctly.
4. Existing APIs remain stable for callers (neighbors, computeSynergyBonus, computeBusinessIncome, computeIncome, applyIncome).
5. Unit tests cover edge/corner/middle cells, range extension, and asymmetry when only one business has upgraded range.
6. Main Street regression suite passes (npm test and npm run build).
7. Documentation explains the grid-distance adjacency model and migration notes for future balancing.

Constraints

• Keep the streetGrid storage shape as a 10-slot array for compatibility.
• Preserve deterministic behavior and avoid randomization changes.
• Do not introduce breaking changes to scene or transcript formats.

Implementation approach

• Implement index<->cell mapping helpers inside MainStreetAdjacency.ts.
• Recompute neighbors(index, range) from Manhattan distance over 2x5 coordinates.
• Update adjacency tests in tests/main-street/adjacency.test.ts to match the new geometry.
• Update docs (docs/main-street/prd-milestone-4.md) with the grid-distance rule.

Related work

• Parent milestone: Main Street: PRD Milestone 4 -- Visual Polish, Animation, and Audio (CG-0MM4RF91E1LR5RSY)
• Discovered from: Unify card dimensions across UI (CG-0MO4NFMY4005DK66)