Fix CI failures: pyarrow guards, DSL API contract drift, UQ reducer keying

[Copilot]

Three dominant failure clusters in CI (71491626052) after the coverage/test additions in the previous PR.

Changes

Missing pyarrow in CI

• tests/test_roundtrip_invariants.py: Guard TestArrowRoundtripZeroLoss with @pytest.mark.skipif(not _PYARROW_AVAILABLE, ...) — skips cleanly when pyarrow absent
• .github/workflows/tests.yml: Add pyarrow to primary test job install; bump coverage job to .[tests,arrow]

DSL/AST contract drift

Fixes 'Query' object has no attribute 'summary', 'list' object has no attribute '_to_ast', and Target.NODES in ['nodes', 'edges'] failures:

• py3plex/dsl/ast.py: Add Query.summary() returning a stable structural dict with a 16-char ast_hash (SHA-256 of canonical JSON AST); change Target to class Target(str, Enum) so string membership tests work
• py3plex/dsl/builder.py: from_layers() now accepts list[str] → converts to LayerExpr; non-string elements raise ValueError

# Before: AttributeError: 'Query' object has no attribute 'summary'
q = Q.nodes().where(degree__gt=5)
print(q.to_ast().summary()["ast_hash"])  # "a3f8c2e1b9d4..."

# Before: 'list' object has no attribute '_to_ast'
Q.nodes().from_layers(["social", "work"])  # now works

assert_network_semantic_equal layer comparison

get_layers() returns visualization objects, not layer names — switched to .layer_names / .layers with a node-attribute fallback.

UQ reducer keying

MockReducer in tests/test_uncertainty_runner.py defined a broken @property def __class__ to fake __name__. Replaced with a plain .name attribute that runner.py's getattr(reducer, "name", None) already expects.