Document set comprehension lowering and backend wire representation

[gordonwatts]

Motivation

• Clarify how set comprehensions are handled by the syntatic-sugar transformer so users and backends understand when a set is lowered to a literal and when it fails.
• Explain the exact AST nodes that are emitted and sent over the func_adl wire so backends can rely on a stable, backend-agnostic representation.

Description

• Implemented lowering for set comprehensions in func_adl/ast/syntatic_sugar.py via visit_SetComp, which inlines literal iterables to ast.Set, emits set() for empty results, and raises ValueError (including ast.unparse(node)) for non-literal iterables.
• Added tests in tests/ast/test_syntatic_sugar.py verifying literal set lowering, empty-literal lowering to set(), and error behavior for non-literal iterables.
• Updated documentation in README.md and docs/source/generic/query_structure.md to document the syntactic-sugar behavior for set comprehensions and to describe what AST form (ast.Set or set() call) is sent to the backend when .value() is called.

Testing

• Ran pytest tests/ast/test_syntatic_sugar.py and all tests passed (45 passed).
• Ran black --check func_adl tests and it completed with no formatting changes required.
• Ran flake8 func_adl tests with no linting issues reported.

---

Codex Task