Scope VGI316 to table functions only (fix scalar/aggregate false positives)

[rustyconover]

Problem

VGI316 (array-argument-could-be-table) fired on scalar and aggregate functions, but its only suggested remedy — "expose it as a table function that takes a table/relation input, so callers pass a subquery (FROM …)" — is structurally impossible for a scalar/aggregate function. A scalar function returns one value per row and cannot take a relation as an argument. Flagging them was a false positive with no achievable fix.

The motivating case: ortools.main.max_flow_value(arcs) — a SCALAR function returning BIGINT — was flagged identically to the real table-function solvers.

Fix

• targets narrowed from (SCALAR_FUNCTION, AGGREGATE, MACRO, TABLE_FUNCTION) to (TABLE_FUNCTION,).
• check() now iterates the existing _iter_table_functions(ctx) helper (used by every other table-function rule in the module) instead of iter_all_functions().
• MACRO was also dropped: DuckDB macros take scalar expressions, not relations, so the "pass a subquery FROM…" advice is equally impossible for them.
• The single-array-argument condition (len(array_args) != 1 continue) is unchanged.

This change only ever removes findings (it narrows targets), so it cannot introduce a new finding on any worker.

Verification against vgi-ortools

Before: 6 VGI316 warnings. After: 5.

function	kind	before	after
max_flow_value(arcs)	scalar	fired	removed (false positive)
max_flow(arcs)	table_function	fired	still fires
solve_assignment(cost)	table_function	fired	still fires
solve_jobshop(tasks)	table_function	fired	still fires
solve_knapsack(weights)	table_function	fired	still fires
solve_tsp(distance)	table_function	fired	still fires

The 5 residual findings are genuine table-function cases (whether ortools should reshape/document them is a separate decision) — the deliverable here is that the scalar false positive is gone.

Gates

ruff format / ruff check / mypy / pydoclint / pytest (505 passed) / gen_rules_doc.py --check all green. RULES.md regenerated; VGI316 unit test updated to assert a scalar/aggregate with a matrix arg is NOT flagged while a table function still IS.

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