feature: unbounded world size -- chunk-based worldgen limited by RAM, not menu slider

[abix-]

Type

• feature (architecture / worldgen)

Problem

World size is currently capped by a menu slider (8K-64K pixels). Players who want Factorio-style exploration are limited to a fixed boundary. The world should grow as the player explores, limited only by available RAM.

Design Direction

Replace the fixed-size world grid with a chunk-based system where terrain is generated on demand as entities or the camera move into unloaded regions.

Key Challenges

1. WorldGrid: currently a flat Vec<WorldCell> sized at init. Needs chunk-based storage (HashMap<ChunkCoord, Chunk>) with lazy generation.
2. GPU compute: spatial grid is a fixed 256x256 grid uploaded each frame. Needs viewport-relative or hierarchical approach.
3. Pathfinding: HPA* cache assumes a fixed grid. Needs incremental chunk loading/unloading for the abstract graph.
4. Tilemap rendering: Bevy tilemap chunks need dynamic load/unload based on camera position.
5. EntityMap spatial buckets: currently sized for fixed world bounds. Needs dynamic resizing or chunk-aligned buckets.
6. Save/load: must serialize only loaded chunks + entity positions. Unvisited chunks regenerate from seed.
7. Town placement: raider towns need to spawn at appropriate distances as the player expands, not all at world init.

Factorio Parallels

• Chunks generated on demand (16x16 tile chunks in Factorio)
• Deterministic generation from world seed -- same seed = same terrain
• Only active chunks (near players/entities) are simulated
• Inactive chunks are frozen or unloaded

Scope Estimate

This is a multi-week architectural change touching nearly every system. Recommend breaking into slices:

1. Chunk storage format + lazy terrain generation
2. Camera-driven chunk loading/unloading
3. Pathfinding adaptation (incremental HPA* rebuild)
4. GPU buffer adaptation (viewport-relative spatial grid)
5. Dynamic town/raider spawning at world edges
6. Save/load for chunk-based worlds
7. Remove world size slider, add seed input

Acceptance Criteria

• World generates new terrain as camera/entities move beyond current bounds
• No fixed world size limit (grows until RAM is exhausted)
• Deterministic from seed (same seed = same world)
• Pathfinding works across chunk boundaries
• Save/load preserves explored chunks, regenerates unvisited from seed
• Performance maintains 60 UPS at 50K NPCs within loaded chunk radius

Status

Waiting -- needs design discussion and slice planning before implementation begins.
This is a foundational architecture change, not a simple feature add.

[abix-]

Blocked: needs a spec doc at docs/unbounded-world.md before this can move to ready. The issue body outlines challenges and slice breakdown but does not specify behavior, data model, edge cases, or acceptance criteria at implementation level. Per the new feature spec requirement in ai-collab-workflow.md.