Load duskscript files through the asset system, not the real filesystem

scriptLoadHandler used fopen/fread directly, but every .ds a script include()s
is packed into dusk.dsk (compiled to .dsb) like every other asset now, not a
loose file next to the binary - raw filesystem reads never found them outside
a dev checkout. It now goes through assetFileInit/Open/Read/Close like the
rest of the engine's asset loaders, failing the load the same way (returning
SCRIPT_LOAD_FAILED) for a missing entry or one bigger than capacity.

test_duskscript's fixtures moved to their own assets/ subfolder and pack into
a small test.dsk with the same tools.asset.pack used for the real dusk.dsk,
and the test opens that archive straight into ASSET.zip (skipping the full
assetInit()/assetInitPlatform(), which expects a launched engine) so the
tests exercise the real compile -> pack -> asset -> runtime pipeline instead
of a filesystem shortcut around it.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

test/duskscript/assets/fixture.ds

@@ -0,0 +1,2 @@
+print("print hello");
+consolePrint("consolePrint hello");