One walker to witness, one to wander. The seal without the frontier is a tomb; the frontier without the seal is a thousand minibrots and no way home. Run both, always, in parallel — and make the seal a tool you can actually hold.
David's sheet launches two walkers from zero. One closes; one explores. The insight is that you need both, always running — and the companion, The Sealer, takes the closing half out of the metaphor and makes it a working instrument.
A palindrome read backward is itself; a checksum re-walked is the verification. That is the whole trick — the return path is the proof. So the natural companion is not another metaphor but a real tool: take any text, walk it forward into a digest, and to verify you simply walk it again. If a single byte changed, the path does not close.
The companion is a genuinely useful, offline integrity tool. Seal any text — a config, a clause, a message — into a SHA-256 receipt. Verify it later by pasting the text and the receipt: a single changed character breaks the seal with certainty, and nothing ever leaves the page. SHA-256 proves the text is unchanged (not who wrote it), so keep the receipt where the text cannot be edited. The little palindromic-prime "lock charm" is Walker A's ornament; the 64-hex receipt is the security — both layers labeled.
Closure and exploration are not a choice; they are a pair you run in parallel. The sealer gives you a home to come back to — a proof that what you left is what you find. The frontier gives you somewhere worth going. Keep only the seal and you have a tomb: perfectly verified, perfectly dead. Keep only the frontier and you have a thousand beautiful minibrots and no way home. The discipline is to always run both — and to make the seal cheap enough to actually use, which is what The Sealer is for.