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feat: improve header sync (#7421)

Description
---
When we have a valid PoW equal to or higher than what we have or what
has been advertised by the sync peer, we need to commit the headers, so
any downstream block sync error does not invalidate the headers that
have been downloaded.

_**Edit:** When block sync fails, we also make sure to swap to the best
PoW chain, also preserving banked headers if we do not need to reorg._

Closes #7342

Motivation and Context
---
See #7342

How Has This Been Tested?
---
System-level tests passed:
- Synching a previously synced node, a couple of days old.
- Syncing from scratch, encountering multiple block-sync failures, but
using the good PoW headers already banked.
```rust
   51952: 2025-08-18 16:07:02.097680100 [c::bn::block_sync] DEBUG Validating block body #22624 (PoW = RandomXTari, input(s): 1, output(s): 54, kernel(s): 2, latency: 108.00µs)
   51961: 2025-08-18 16:07:02.111648900 [c::bn::block_sync] DEBUG Validated in 14ms. Storing block body #22624 (PoW = RandomXTari, input(s): 1, output(s): 54, kernel(s): 2)
   51962: 2025-08-18 16:07:02.111668100 [c::bn::block_sync] TRACE Hash: <a class=hub.com/tari-project/tari/commit/91a1fbccc1f94e2aa93b6efdaf9c3fbf16d0588c">91a1fbccc<a href="https://github.com/tari-project/tari/commit/84c81d129edc8d90adeec2eed2e207a61b44bef4">3458b7ef444e605f62375077
   54024: 2025-08-18 16:11:06.389482300 [c::bn::block_sync] WARN  Peer did not supply all the blocks they claimed they had: Their claim - height: 74721, accumulated difficulty: <a class="double-link" href="https://github.com/tari-project/tari/commit/4089235547434023187024157117581153168163">408923554</a><a href="https://github.com/tari-project/tari/commit/84c81d129edc8d90adeec2eed2e207a61b44bef4">715651441378. Our status after block sync - height: 22624, accumulated difficulty: </a><a class="double-link" href="https://github.com/tari-project/tari/commit/1547640955349056263010512134240412613668">154764095</a><a href="https://github.com/tari-project/tari/commit/84c81d129edc8d90adeec2eed2e207a61b44bef4">889130919000
   54028: 2025-08-18 16:11:06.390047900 [c::bn::block_sync] WARN  Block sync failed - best header: 74723/1bcf293971df2d6c888a6589dca3a323b, best block: 22624/91a1fbccc1f94e2aa93b6efdaf9c3fbf16d0588c3458b7ef444e605f62375077. No more sync peers available: Block sync failed
   54109: 2025-08-18 16:11:11.596105400 [c::bn::header_sync] DEBUG Starting header sync.
   54110: 2025-08-18 16:11:11.59617... (continued)

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