[barrulus]

HighMap fetches LightUSD from its moving main branch. LightUSD now defaults to its next implementation, which does not provide the lightusd_static target expected by HighMap; the upstream macOS build fails compiling export_usd.cpp with core/animatable.hh missing.

Declare LightUSD before HighMap so CMake uses the same compatible revision already pinned in nix/highmap-sources.nix. This applies to all CMake builds.

Validation:

• Configured a CMake probe using HighMap’s actual LightUSD dependency settings and the pinned source; verified lightusd_static exists and compiled all USD headers used by the failing source plus a Stage/GeomPoints API probe.
• Verified the tested source hash matches the Nix pin.
• CI automation suite: 22 tests, 7 skipped, no failures.
• git diff --check passed.

Full macOS validation requires the PR build. The separate Linux apt DNS failure in the latest main run is unrelated to this change.