Support Sponge Schematic version 2 and 3 (.schem)

[Qalipso]

Builds on #41, which this branch contains as its first commit.

Problem

builder_read_schematic rejects any schematic whose Version tag is above 1:

if (version > 1)
{
    log("Schematic error", "Unsupported format, version too high")
    return false
}

WorldEdit and the other current tools write version 2 and version 3 files with a .schem extension, so they cannot be opened at all — and .schem is not in the scenery file dialog, asset_load or asset_exts either.

Format differences handled

	version 1	version 2	version 3
root compound	Schematic	Schematic	"" → Schematic
palette	Palette	Palette	Blocks.Palette
block indices	BlockData	BlockData	Blocks.Data
block entities	TileEntities	BlockEntities	Blocks.BlockEntities

Block entity objects carry Id and Pos in all three versions, so builder_read_schematic_tile_entities works unchanged once the right list is picked up.

Change

• builder_read_schematic resolves the palette, block data and block entity list per version.
• res_load_scenery unwraps the version 3 root compound.
• The format check compares the tag type of Blocks rather than its value, so a version 3 Blocks compound is recognised while a legacy MCEdit Blocks byte array still takes the legacy path.
• .schem added to the scenery file dialog, asset_load and asset_exts.

Verification

• The new GML was transliterated line for line and run against real gzipped NBT files: versions 1, 2 and 3 with palettes of 64, 300 and 4096 entries all decode with zero mismatches and resolve the correct block entity list, and a legacy MCEdit schematic is still detected as legacy. A deliberate mutation of the decode loop was caught.
• CppGen was run over GmProject and reports Success!.
• As with the parent PR I could not build and run the application, so this has not been exercised through the UI.

CppProject/Generated is left untouched and needs a CppGen run to pick this up.

🤖 Generated with Claude Code