`crucible module build` runs the full pipeline (module + HAK producer +
topdata). A module-only project consumes prebuilt HAKs and has no
`paths.assets`, so the HAK stage crashed on the unset assets dir: the
music-credits walk hit `filepath.WalkDir("")` (`lstat : no such file or
directory`) and asset collection hit `filepath.Rel(root, "")`.
Guard both on an unset `AssetsDir()`: an absent assets tree simply means
no authored credits and no asset files to collect, so the HAK stage
no-ops cleanly while topdata generated-2DA assets still build. Honors
spec Decision 3 (canonical `crucible module build`, no `build-module`).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Reviewed-on: #17
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>