GotA browser asset packer ignores POLYWORLD_ART #103

What happens. coworld/tools/build_replay_viewer.nim and src/polyworld/emscripten.nims
honour POLYWORLD_ART and pass it to the GotA packer (pack_assets ART CACHE). The packer
still reads the character library from the default sibling checkout, so a build with the art
anywhere else fails:

cannot open ../polyworld_art/characters/chargen/manifest.json

Repro.

1. Check out Metta-AI/polyworld_art at the coworld/assets.json revision somewhere other than
   <repo>/../polyworld_art, and make sure that sibling path does not exist.
2. export POLYWORLD_ART=/path/to/polyworld_art
3. bash coworld/gota/tools/build_replay_viewer.sh /abs/output

Cause. examples/gods_of_the_arena/assets.nim builds its asset list from DataRoot
(src/polyworld/common.nim), which is the constant "../polyworld_art" in native builds, so
generatedCharacterAssets() and other declarations resolve relative to the working directory
rather than the packer's art-root argument. ChargenLibrary in src/polyworld/chargen.nim is a
similar compile-time sibling path.

Workaround. Symlink the art checkout to <repo>/../polyworld_art.

Suggested fix. Resolve asset declarations against the art root passed to the packer (or
read POLYWORLD_ART in DataRoot for native tools), and add a build check that runs the packer
with the art outside the sibling path.

Found while building the replay viewer for #102.

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