feat: warn instead of refusing on checks that only raise under the built-in policy

Under the built-in policy, arithmetic that only fails at its 38-digit
limit, date arithmetic and nanosecond comparisons that only fail outside
DuckDB's timestamp range, and a naive timestamp joined with a local one
that only fails on digits below the microsecond now convert with a
warning instead of being refused. Their converted SQL raises an error
rather than return a different value. --strict refuses them, and
--loud-divergences-only keeps them while it still refuses the clock.

Last-30-days filters on CURRENT_TIMESTAMP() convert too, and SQ0003 on a
function operand now points at a cast. The warnings give DuckDB's exact
timestamp range and explain the UTC session the clock relies on.

The corpora record 132 statements that now translate and 3 whose
refusal moves to a later operand.

Author: kvlonge

README.md

@@ -157,7 +157,8 @@ renumbered.
 ### Policy
 
 `convert`, `function`, `census` and `index` read `sqlquack.toml` (or `--policy`); without one they
-use the built-in policy, which refuses every known divergence, as `--builtin-policy` asks for.
+use the built-in policy, as `--builtin-policy` asks for: it refuses every known divergence except the
+clock and arithmetic that raises at its range limit, which convert with a warning.
 They share the policy flags: `--policy`, `--builtin-policy`, `--strict`, `--utc-session`,
 `--confirm-declared-offsets`, `--python-runtime` and `--loud-divergences-only`, which relies only
 on divergences whose converted SQL raises rather than returns another value. Every command with a