docs(prover): the hash-metrics caveat expired with the merge

The prove-and-retire design doc said the verify-hash column in its
measurements table could not be reproduced on the branch, because the
`hash-metrics` feature lived only on #987. The merge of main brings #987,
so the feature is on the branch now and the claim is stale in the direction
that matters: it tells a reader the number is unreproducible when it is not.

Says what is actually true instead, including the part that is still
missing — the CLI has no passthrough for the feature, so reproducing the
column from `cli trace-build` needs one line in its Cargo.toml. Left for
the branch's author rather than added here; this merge is not the place to
grow the CLI's feature surface.

Author: MauroToscano

docs/prove_and_retire_design.md

@@ -131,9 +131,10 @@ Each run prints the pass timings, `Trace build (prove-and-retire): N tables, T s
 
 Verification on its own: `cli verify <proof> <elf>` for a per-table proof
 written with `--output`. The `hash-metrics` feature that prints a verify's
-keccak count comes from #987, which is **not on this branch** — the verify-hash
-column in §6 was measured on a tree that has it, and cannot be reproduced here
-until this branch is rebased onto it.
+keccak count arrived with the merge of `main` (#987), so the verify-hash column
+in §6 can now be reproduced on this branch — through `lambda-vm-prover` or
+`crypto` directly. The CLI has no passthrough for it yet, so enabling it from
+`cli trace-build` needs one adding to `bin/cli/Cargo.toml`.
 
 ## 5. What verifies, and with what