Move storage_impl.h and storage_traits.h into the vocabulary layer's tiers

[trueqbit]

Summary

Neither storage_impl.h nor storage_traits.h is about the storage object. Both compute over the schema, the tuple of mapped database objects. This PR places their contents by the rules in docs/internals/vocabulary-layer.md and gives them schema_* names instead of storage_*, as planned there.

Was	Now	Tier
storage_traits::storage_mapped_columns_impl	column_field_types_t, in vocabulary/algorithms/column_field_types.h	vocabulary algorithm: computes over a single table definition
storage_traits::storage_mapped_column_expressions_impl	column_field_expressions_t, same file	vocabulary algorithm
storage_traits::storage_mapped_columns	schema_mapped_column_field_types, in schema/algorithms/table_lookup.h	schema algorithm: looks up the table first
storage_traits::storage_mapped_column_expressions	schema_mapped_column_field_expressions, same file	schema algorithm
lookup_table_name() (declared in table_lookup.h, defined in storage_impl.h)	defined in schema/algorithms/table_lookup.h	schema algorithm
find_column_name(), materialize_column_pointer(), is_column_unique()	schema/algorithms/column_lookup.h	schema algorithm
tables_index_sequence, views_index_sequence, foreign_keys_count()	schema/algorithms/table_filters.h	schema algorithm
column_field_expression_t (schema/column.h)	the projection member_pointer_type_t, in vocabulary/projections/nested_types.h	projection

• column_field_expression_t had only one user, the column expressions computation. Replacing it with a plain projection means the new vocabulary algorithm doesn't depend on schema/column.h.
• column_result.h and column_expression.h now reach schema/column.h later in the amalgamation than before, because storage_traits.h no longer pulls it in. The column names they and the headers before column.h use are forward-declared in vocabulary/node_fwd.h and traits/grammar_traits_fwd.h.
• The vocabulary doc describes the new files, and its open-work item for storage_traits.h is removed as done. AGENTS.md no longer lists storage_impl.h.

Test plan

• tests/static_tests/select_return_type.cpp checks schema_mapped_column_field_types (formerly storage_mapped_columns), and adds checks for schema_mapped_column_field_expressions on a mapped and an unmapped type.
• select(asterisk<T>()) and the asterisk column result tests exercise both through column_result_t / column_expression_type.
• The find_column_name tests (tests/storage_tests.cpp, tests/logger_tests.cpp) and the CTE tests cover the column lookups. The foreign key tests cover foreign_keys_count().

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