Closes the second drift-evolution gap from #328. PR #332 lifted is_safe_type_widening + alter_column_type_sql onto the SqlDialect trait with default impls preserving Databricks/Spark semantics. This PR adds the BigQuery overrides + the runtime wiring to actually emit AlterColumnTypes SQL.

Engine changes

• BigQueryDialect::alter_column_type_sql overrides the default ANSI form with BQ's required ALTER COLUMN x SET DATA TYPE y. The default ALTER COLUMN x TYPE y shape returns Expected keyword DROP or keyword SET from BigQuery.

• BigQueryDialect::is_safe_type_widening declares a strict BQ-specific allowlist:

  • INT64 → NUMERIC (lossless: INT64 fits in NUMERIC precision 38)
  • INT64 → BIGNUMERIC (lossless)
  • NUMERIC → BIGNUMERIC (strict precision widening)

  Excluded by design:

  • … → FLOAT64: lossy for absolute values > 2^53. BigQuery accepts via SET DATA TYPE but Rocky's "safe" contract is strict (matches the default Databricks/Spark allowlist that omits INT → FLOAT).
  • … → STRING: BigQuery's ALTER COLUMN SET DATA TYPE rejects these with existing column type X is not assignable to STRING even though STRING is lossless at the value level. The default allowlist's "any numeric → STRING" pattern doesn't transfer to BQ. Discovered via live verification — initial draft included these patterns and live runs surfaced the error against stage 3 of the existing smoke test.

• run.rs::process_table adds the DriftAction::AlterColumnTypes branch between DropAndRecreate and the existing added_columns branch. Emits via drift::generate_alter_column_sql (which routes through dialect.alter_column_type_sql) and surfaces as action: "alter_column_types" in the run output. If the same drift round also surfaced added columns, both apply before the INSERT continues.

Smoke test

Extends live/drift/run.sh to a four-stage flow:

1. Initial 4-column source (with score INT64) → replicate clean
2. ALTER source ADD COLUMN region → add_columns action
3. DROP+CREATE source with id INT64→STRING → drop_and_recreate (unsafe per BQ allowlist)
4. DROP+CREATE source with score INT64→NUMERIC → alter_column_types (safe widening)

==> stage 4 drift: action=alter_column_types OK (["column 'score' widened INT64 → NUMERIC"])
==> target customers.score is now NUMERIC (alter_column_types took effect)

Idempotent across consecutive runs.

Test plan

• cargo test -p rocky-bigquery --lib — 68 passed (8 new dialect tests)
• cargo clippy -p rocky-bigquery -p rocky-cli -p rocky-core --all-targets -- -D warnings clean
• cargo fmt -p rocky-bigquery -p rocky-cli -p rocky-core --check clean
• live/drift/run.sh against the BQ sandbox — exits 0, all 4 stages pass
• Two consecutive runs both pass (idempotency)