Fix excessive memory use with large external-level projects

[GameGuyy]

Fixes #1172

Large projects with external levels were crashing the renderer because Electron's V8 heap is capped around 4GB (pointer compression), and every level was kept in memory 3 times: the live data, a parsed LevelJson object, and the pretty-printed JSON string. With the crasher project from the issue this goes over the cap before loading even finishes.

Changes:

• For external-level projects, the level JSON cache no longer keeps the string/object in RAM. It just points at the level's own .ldtkl file on disk, and reads it back only when actually needed.
• Saving doesn't wipe and rewrite the whole levels folder anymore. Unchanged levels are skipped entirely, renamed levels get their file copied, and only dirty levels are re-serialized — one at a time, so the peak memory during a save is a single level. Orphaned .ldtkl files are cleaned up after the save succeeds.
• The main project JSON is built with layer instances skipped instead of serializing everything and stripping fields afterwards.
• Crash backups go through the same one-level-at-a-time path.

Single-file projects keep the old in-memory cache, nothing changes for them.

Tested with the crasher project from the issue (55 levels, ~276MB): loads and saves fine now, renderer stays under 1GB where it used to hit 4GB and die. Also checked: saving without edits touches only the main .ldtk file, editing one level rewrites only that level's file, level rename/delete, save as to another folder, 0.9.3 project migration, and close/reopen round-trips (multi-world sample included).

[GameGuyy]

Just wanted to bump this. It fixes a renderer OOM crash on large external-level projects (#1172). Tested against the crasher project from that issue: renderer memory stays under 1GB now, where it previously blew past the ~4GB V8 heap cap and crashed before load even finished. Happy to address any review feedback.