feat(spv): cross-check recent headers with a witness server

Blocks above the newest checkpoint cannot be anchored, so the header from
the primary server is now compared with the one from a second server taken
from the hardcoded bootstrap list. A disagreement at 3+ blocks of depth
aborts the sync; a shallower one leaves the proof unverified until the next
sync; with no reachable witness behaviour is unchanged. A bounded
proof-of-work check was evaluated and rejected. See ADR 0042.

SECURITY.md

@@ -21,7 +21,7 @@ It does **not** protect against:
 
 This wallet is an SPV client, not a full node. It validates:
 
-- Block headers: `SkipPowValidation` is enabled because LWMA difficulty cannot be recomputed client-side, so proof of work is never checked. Instead, every header used for a Merkle proof is hash-chain-linked (prev-hash) **up to the nearest hardcoded checkpoint at or above its height**, and the header at that checkpoint height must hash to the checkpoint exactly (`Core/Chain/ChainProfiles.cs`, enforced in `Core/Spv/WalletSynchronizer.AnchorToCheckpointAsync`, ADR 0040). The direction matters: with PoW skipped, a chain built *from* a checkpoint *up to* a header proves nothing, because anyone can fabricate such a chain; a chain ending in a known hash cannot be forged without inverting SHA-256. Mainnet checkpoints run every 20,000 blocks from 20,000 (so even the oldest blocks are covered) and are extended at each release. **Known limit:** a transaction in a block *above the highest checkpoint* has no checkpoint to anchor to, so only its Merkle proof against the header the server supplied is checked there; the window is the blocks mined since the last release. The wallet flags such transactions as **not anchored** in the history list and in the transaction detail (ADR 0041), so the reduced guarantee is visible instead of silent. Testnet/regtest have no checkpoints yet, so headers there are trusted at face value
+- Block headers: `SkipPowValidation` is enabled because LWMA difficulty cannot be recomputed client-side, so proof of work is never checked. Instead, every header used for a Merkle proof is hash-chain-linked (prev-hash) **up to the nearest hardcoded checkpoint at or above its height**, and the header at that checkpoint height must hash to the checkpoint exactly (`Core/Chain/ChainProfiles.cs`, enforced in `Core/Spv/WalletSynchronizer.AnchorToCheckpointAsync`, ADR 0040). The direction matters: with PoW skipped, a chain built *from* a checkpoint *up to* a header proves nothing, because anyone can fabricate such a chain; a chain ending in a known hash cannot be forged without inverting SHA-256. Mainnet checkpoints run every 20,000 blocks from 20,000 (so even the oldest blocks are covered) and are extended at each release. **Known limit:** a transaction in a block *above the highest checkpoint* has no checkpoint to anchor to, so only its Merkle proof against the header the server supplied is checked there; the window is the blocks mined since the last release. The wallet flags such transactions as **not anchored** in the history list and in the transaction detail (ADR 0041), so the reduced guarantee is visible instead of silent. For those blocks the wallet also cross-checks the header with a second server taken from the hardcoded bootstrap list (never from announced peers): a different header at a safe depth (3+ blocks) aborts the sync, a shallower disagreement leaves the proof unverified until the next sync, and with no reachable witness only the Merkle proof applies (ADR 0042). This raises the bar from one lying server to two; it does not help against two colluding servers or an attacker in front of both. Testnet/regtest have no checkpoints yet, so headers there are trusted at face value
 - Transaction inclusion in a confirmed block (Merkle branch proof, mandatory for every confirmed transaction — see `Core/Spv/MerkleProof.cs`)
 
 It does **not** validate: