techdesign(pipeline): design hash-based incremental chunk skip

[nigel-hall-codes]

Design a system to skip regenerating chunks whose inputs haven't changed since the last run. Key questions to answer in the design doc: What inputs determine chunk staleness (graph crop, heightmap crop, generation settings, generator version)? Where is the hash stored (sidecar file, manifest, prefab asset)? What granularity (per-chunk vs per-stage)? How does the user force a full regenerate? Output: design.md with chosen approach + list of implementation issues.

Discovered from #55.

[nigel-hall-codes]

Design draft: sdlc/#60/design.md

Summary of decisions:

Staleness inputs — OSM mtime + CSV mtime + chunkSizeMeters + col/row + roadWidthMultiplier + defaultBuildingHeight + heightmapResolution + GeneratorVersion constant. FNV-1a hash (no crypto dependency, mtime-based like the existing ElevationParser cache).

Storage — chunkHashes object added to the existing manifest.json. Previous manifest read at run start; updated hashes written at end. No sidecar files, no AssetDatabase round-trips.

Granularity — per-chunk, all-or-nothing. Skip the entire chunk loop body. Skipped chunks load their existing prefab and instantiate it under mapRoot so the scene stays complete.

Force full regenerate — no new button. "Clear Generated Assets" already deletes manifest.json, which collapses all hashes → full regen on next press.

5 child implementation issues listed at bottom of the doc.

Branch: sdlc/#60-techdesign