fix(spv): anchor headers up to a checkpoint above them

With proof of work skipped, chaining headers from a checkpoint up to a
transaction's block proves nothing: a server can fabricate a header and
fake headers after a real checkpoint. The header must now hash-chain up to
the nearest checkpoint at or above its height, whose hash is hardcoded.
This also anchors heights below the first checkpoint. The cached anchor
state is renamed AnchoredDownTo and caches without it are re-verified
once. Blocks above the newest checkpoint remain a documented limit.
See ADR 0040.

docs/adr/README.md

@@ -43,6 +43,7 @@ One ADR per feature or significant design decision. Format: `template.md`. Workf
 | 0037 | [Derive an address private key only when it is revealed](0037-derive-address-private-keys-on-reveal.md) | Security | Accepted |
 | 0038 | [CLI password sources and explicit plaintext storage](0038-cli-password-sources-and-explicit-plaintext.md) | Distribution | Accepted |
 | 0039 | [Server fee estimates, relay floor and a high-fee warning](0039-server-fee-estimates-and-relay-floor.md) | Wallet | Accepted |
+| 0040 | [Anchor headers up to a checkpoint above them](0040-anchor-headers-up-to-a-checkpoint.md) | SPV | Accepted |
 
 ADRs `0002` onwards were written retroactively on 2026-11-03 from the code, comments and changelog. 
 If one misstates the original reasoning, fix it in a pull request.