verify: flag a multi-file question answered from only one table

Extends the join-hint work in #58 (`shared_column_hints` in the schema digest) with a check on the *answer* side, not just the prompt side.

**Mechanism.** When a question's wording references vocabulary from column names/values that live in ≥2 tables (the same signal `shared_column_hints` already computes for the prompt), but the answer's evidence trail only touched one table — flag it as a soft warning ("this question looked like it needed a join; the answer didn't use one"). Doesn't block the answer, just marks it for review, the same tier as the existing soft warnings in `verify.rs`.

**Why:** multi-file questions are exactly the class #58 was built to help with; this is the deterministic check that catches when the steering didn't take, instead of only finding out from a benchmark run months later.

Related: #58, #67.

Start with verify.rs and the shared_column_hints logic in the schema digest, then inspect related issues #58 and #67 for existing soft-warning behavior. Trace how the answer evidence records touched tables; done when a prompt spanning at least two tables gets a same-tier warning when the evidence names only one table, without blocking verification.

Written by the indexing model from the issue text.