fix: correct isometric tileset rendering

[dxcvvxd]

Purpose of change

Fixes broken isometric tileset rendering (black voids between tiles, flat z-level stacking) caused by debug-zeroing of tileset JSON values and wrong iso tile count / screen-row calculations.

Describe the solution

1. Remove 3 debug-zeroing lines in tileset_loader::load() — ts.zlevel_height, ts.prevent_occlusion_min_dist, ts.prevent_occlusion_max_dist were set to 0/-1 after being read from JSON, rendering z-level and occlusion features non-functional.
2. Fix iso screentile_width/height — iso mode needs ceil(width*2/tw)+1 / ceil(height*4/tw)+1 instead of ortho formula. Without this, not enough tiles are queued for rendering → black voids.
3. Add per-z-level screen_row offset for iso — apply (center.z - tile.z) * zlevel_height * 4 / tile_width to screen_row so z-levels stack vertically instead of rendering flat.
4. Fix get_window_tile_counts() iso formula — same scaling fix as Coolthulhu fix null tool #2, controls minimap and offscreen area tile counts.

Testing

• Loaded iso tileset with each fix individually to isolate the effect.
• Verified non-iso tilesets are not affected (ortho code paths unchanged).
• Build clean.

Checklist

• This PR used AI assistance.
  • I added an Assisted-by: trailer to every AI-assisted commit.
• This is a C++ PR that modifies JSON loading or behavior.

Build Artifacts

PR build for commit 61910fc (style(autofix.ci): automated formatting) on 2026-07-20 17:15:53

• ✅ Linux tiles — download
• ✅ Windows tiles — download
• ✅ macOS tiles — download
• ✅ Android tiles — download

[autofix-ci]

Autofix has formatted code style violation in this PR.

I edit commits locally (e.g: git, github desktop) and want to keep autofix

1. Run git pull. this will merge the automated commit into your local copy of the PR branch.
2. Continue working.

I do not want the automated commit

1. Format your code locally, then commit it.
2. Run git push --force to force push your branch. This will overwrite the automated commit on remote with your local one.
3. Continue working.

If you don't do this, your following commits will be based on the old commit, and cause MERGE CONFLICT.

[scarf005]

1. remove all the fprintfs
2. please add screenshots of isometric tileset working correctly to PR body