Move function.h under ast/ as app_function.h and grammar-match the function call nodes

[trueqbit]

Summary

Two commits, both about the application-defined function machinery and its place in the vocabulary layer.

1. dev/function.h → dev/ast/app_function.h. The header holds the call node of an application-defined function together with the definition machinery that generates it (func<UDF>, "name"_scalar) — the same shape as ast/builtin_function.h, so it belongs next to it. The definition object and the call node are named after it, app_function<UDF> and app_function_call<UDF, Args...>, mirroring builtin_function / builtin_function_call; function and function_call said nothing in internal. "Application-defined" is SQLite's own term (sqlite3_create_function). The header specializes the operand and grammar traits in place, so it is now registered in node_definitions.h, which it had been missing from.

2. is_app_function_call + is_builtin_function_call, and the rule behind them. The consumers of the app-function call node — ast_iterator, column_result_t, node_tuple, statement_serializer, udf_existence_checker — pattern-matched and decomposed the concrete node. They now grammar-match on a new is_app_function_call trait and read through udf_type_t / args_tuple_t, and no longer include the node header at all. The built-in side gets the same treatment: is_builtin_function_v (a primary-false template with a pattern partial specialization per call node) becomes is_builtin_function_call_v, one closed definition over is_base_template_of<builtin_function_call, T> that also covers the derived aggregate node; is_operator_argument_v is enabled on the grammar trait for both. The rename says what the trait classifies: the call node, not the definition object.

docs/internals/vocabulary-layer.md gains two sections stating the rule — a node's own trait is one closed definition over a type predicate, a joined axis is a SFINAE enabler on the node's grammar trait, and consumers grammar-match and read through the projections rather than pattern-matching the node — and AGENTS.md carries the short form.

No public API changes: func<>, "x"_scalar, the orm_* concepts and storage.create_*_function() are untouched.

Test plan

• tests/static_tests/function_static_tests.cpp: is_app_function_call_v positive for scalar and aggregate call nodes, negative for the UDF type and the definition object; is_operator_argument_v on both nodes.
• tests/static_tests/builtin_function_static_tests.cpp: renamed assertions.
• Full suite compiles and runs locally.

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