[doubleailes]

Fixes #165.

Problem

When an anonymous layer had a subLayers entry that wasn't loaded yet,
LayerGraph::file_stack_discoveries dropped the entry before it reached the
stage's load barrier. An absolute-path sublayer authored on an anonymous layer
therefore never opened. That covers both Stage::insert_layer and authoring on
the anonymous root of Stage::builder().in_memory(...). composition_errors()
stayed empty, so the opinions disappeared without any diagnostic.

Fix

This follows C++. An anonymous layer's resolved path is empty, so
SdfComputeAssetPathRelativeToLayer creates the entry's identifier without an
anchor and the layer stack opens it like any other sublayer. A missing asset
reports PcpErrorInvalidSublayerPath.

The skip is removed, so an anonymous parent's unresolved entry now becomes a
load request like any other parent's:

• An absolute entry opens from disk.
• A missing asset reports UnresolvedSublayer.
• An in-memory layer with the literal name still resolves when it joins,
  through the existing bare-name fallback.

References and payloads authored on anonymous layers already went through this
path, so sublayers now behave the same way. The Stage::insert_layer docs
already described this behavior.

There is one behavior change. A relative entry on an anonymous layer that names
no in-memory layer is now also looked up on disk. If it isn't found it reports
UnresolvedSublayer, as C++ does, where before it was silently ignored.

Tests

• pcp::layer_graph::tests::anon_parent_demands replaces
  anon_parent_no_demand, which asserted the old skip.
• usd::stage::tests::anon_absolute_sublayer covers cases A and B from the
  issue.
• usd::stage::tests::anon_missing_sublayer_reports checks that a missing file
  reports UnresolvedSublayer.

All three tests fail without the fix. cargo test --workspace --all-targets --all-features, cargo clippy --workspace --all-targets --all-features -- -D warnings, cargo doc and cargo fmt --check pass.