feature: market building with trade caravans between factions

[abix-]

Type

• feature (economy / diplomacy)

Problem

There is no way to trade resources between towns. The player's economy is isolated -- surplus resources in one town can't help a deficit in another, and there's no economic interaction with AI factions.

Goal

A Market building enables inter-faction trade via physical caravans that move across the terrain. Players can set up one-time or scheduled (recurring) trades, exchanging resources like food, gold, wood, and stone with other towns that also have a Market.

Scope

Chunk 1 -- Market Building + Trade UI

• New BuildingKind::Market variant + BuildingDef registry entry
• Market building placed in town grid (Economy category)
• Trade UI panel when Market is selected: pick target town, resource type, amount, direction (buy/sell)
• Trade offers listed per-market with accept/reject for incoming offers

Chunk 2 -- Caravan NPCs

• New Job::Trader NPC type spawned by Market buildings
• Caravan NPC carries resources physically across the map using existing pathfinding (A*/HPA*)
• Caravan has a CarriedResources component (similar to CarriedLoot pattern from Stage 18)
• Caravan walks from origin Market to destination Market, deposits resources on arrival
• Vulnerable to raiders while traveling -- killing a caravan drops its goods

Chunk 3 -- Scheduled Trades

• One-time trade: single caravan dispatch
• Scheduled trade: recurring dispatch at configurable interval (e.g., every 2 game-hours)
• Schedule stored as ECS component on Market entity
• Cancel/modify schedules via Market UI
• Trade requires both towns to have a Market building

Chunk 4 -- AI Faction Trading

• AI towns with Markets can propose trades to the player
• AI trade offers appear in the Market UI
• AI evaluates trade value based on resource surplus/deficit
• Ties into Stage 29 (Diplomacy) -- trade reputation affects faction relations

Dependencies

• Stage 26 (Resources) for wood/stone/iron resource types
• Stage 29 (Diplomacy) for faction reputation (Chunk 4 only)
• Existing pathfinding (Stage 20) for caravan movement
• Existing loot carry pattern (Stage 18) for resource transport

Design Notes

The Merchant building already exists but serves as a buy/sell equipment shop. The Market is a separate building focused on resource trade between towns. Consider whether the existing Merchant could be repurposed or if a distinct building makes more sense.

Caravans should use the existing NPC movement system (GPU compute for position, CPU for decision/pathfinding). They don't fight back -- they flee or die if attacked, dropping carried resources as loot.

Acceptance Criteria

• Market building placeable in town grid
• Trade UI: select target town, resource, amount
• Caravan NPC physically walks between towns carrying resources
• Resources transfer on caravan arrival at destination Market
• Caravans are vulnerable to attack (drop resources on death)
• Scheduled trades dispatch caravans at recurring intervals
• cargo clippy --release -- -D warnings clean

[abix-]

Blocked: needs a spec doc at docs/market-building.md before this can move to ready. Must define: trade caravan behavior, pricing model, building placement rules, NPC merchant job interactions, faction diplomacy effects, UI, and edge cases. Per the new feature spec requirement in ai-collab-workflow.md.