feat: recompute computed column rows whose inputs changed

refresh_column fills nulls, so once a row has a value nothing revisits
it: an update to one of its inputs, or a definition change, leaves the
computed value stale for good.

This stamps the column's field metadata with the definition it was
computed under and a per-fragment signature of the input storage it was
read from (input data files and overlays; not the deletion file, since
a delete changes no surviving value). A refresh recomputes every live
row of a fragment whose stamp disagrees with the manifest, then records
what it computed from in a second commit after the fill. A compacted
fragment inherits freshness through the Rewrite lineage when every
fragment it was built from was signed, or was appended since the stamp,
never had an input moved, and left its rows of the product unfilled (a
raw append may supply a value; the product's data is the evidence, and
the null fill covers those rows); otherwise it recomputes. A
column declared before the stamps existed keeps the null-fill contract
on its first refresh, which enrolls it as it stood.

The stamp is a metadata-only commit on the computed columns, so a
materialized view's drift check treats it like the fill. The core lives
in `table::freshness` so a remote refresh can share the contract.

Cargo.lock

@@ -5553,6 +5553,7 @@ dependencies = [
  "serde_json",
  "serde_with",
  "serial_test",
+ "sha2 0.10.9",
  "snafu 0.8.9",
  "tempfile",
  "test-log",