Fix for ignored primvars:crypto_asset

[cpichard]

Changes proposed in this pull request
Issue: primvars:crypto_asset (and similarly resolved inherited primvars) was being ignored when an asset was added through an instanceable reference, causing the Cryptomatte asset AOV to come out empty.

Root cause: The primvar is authored as a constant value on a parent Xform and inherited down to the geometry. When resolved through an instanceable reference, it arrived at the shape as a single-element uniform (per-element) array rather than a true constant value. Arnold declared this as a size-1 uniform array, but then tried to index it per-face, producing an out-of-range error (index N of 1) and silently failing to apply the value.

Fix (libs/common/parameters_utils.cpp): DeclareAndAssignParameter now treats any single-element array as constant user data, unless it's explicitly a "constant array" scope (scope == constantArray). Previously it only did this for scope == constant or the special-cased displayColor attribute — so uniform/varying/indexed single-element arrays fell through and got declared as arrays, which Arnold couldn't index correctly. Now a single value correctly applies to the whole shape regardless of interpolation scope.

Test: testsuite/test_2699 sets up an instanceable reference with an inherited primvars:crypto_asset, and asserts on the render log (no out-of-range/failed-lookup errors) rather than pixel values, since Cryptomatte hashes differ across reading paths.

I tested it with HTOA + nuke cryptomatte.

Issues fixed in this pull request
Fixes #2699

[cpichard]

✅ CI passed (build 11).

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