rpc: dumptxoutset rollback without invalidating blocks (backport of bitcoin/bitcoin#33477)

[chrisguida]

Backport of bitcoin#33477 (fjahr, merged 2026-04-19) to 29.x-knots.

dumptxoutset with rollback currently invalidates blocks down to the target height,
suspends network activity, writes the snapshot and reconsiders the blocks. While it runs
the node is in a state that does not reflect reality, it cannot serve peers or follow the
chain, and a fork at the target height makes it fail. Reviewing an assumeutxo chainparams
entry means running exactly this on a synced node, which discourages people from doing it.

With this change the RPC copies the UTXO set into a temporary database, disconnects blocks
on that copy down to the target height, computes the snapshot from the copy and deletes
it. The node keeps running normally. A temporary prune lock keeps the needed block data on
pruned nodes. The new in_memory=true option keeps the temporary database in RAM (more
than 10 GB on mainnet) and is faster.

Deep rollbacks are slower than before (upstream measured 9m16s vs 3m17s for ~1500 blocks
on disk) but the time no longer depends on the state of the main chain, and the node stays
online.

Adapted to Knots' human-readable dumptxoutset format: the rolled-back dump takes the
same format, show_header and separator arguments as the tip dump. The functional
tests that checked the network-activity handling are dropped (there is nothing to suspend
any more); the upstream fork test and the in-memory case are added, and the sqlite tool
test exercises the rollback path.

Commits:

• rpc: Don't invalidate blocks in dumptxoutset (49d5e83 + fc73601)
• test: Add dumptxoutset fork test (ab9463e)
• test: Extend named pipe sqlite tool test to use rollback (d0fd718)

Motivation: makes verifying the assumeutxo snapshot in #444 a background
operation on any synced node.

Testing done

• Built on Ubuntu 26.04 (g++ 15.2, cmake 4.2.3, boost 1.90) on 2026-10-07.
• test/functional/rpc_dumptxoutset.py and tool_utxo_to_sqlite.py pass (python 3.14).
• Mainnet, on a pruned assumeutxo node (Knots 29.4.2 + this PR + chainparams: add assumeutxo snapshot at height 976000 #444, snapshot loaded at
  976000, tip 976069, 16 cores, NVMe): dumptxoutset rollback=976001 while the node kept
  its 10 peers and followed the chain. On disk: 784 s (copying 167M coins into the temporary
  database is most of it; the copy was 9.1 GB and was removed afterwards). With
  in_memory=true: 473 s. Both runs returned the same txoutset_hash
  (ed7dfbf6388813105dbc7c39bb44ee5503252868c39db8f0d93607dbc342fb98) and nchaintx
  (1417215824). Rolling back to the snapshot base height itself (976000) is refused on such
  a node with "necessary block data is already pruned", because the base block has no undo
  data until background validation reaches it; that check is unchanged from the current code.

[chrisguida]

An assembler-ready copy is at chrisguida/dumptxoutset-rollback-copy-29+knots (same three commits rebased onto the compat_rpc_dumptxoutset_hr merge, the oldest point on 29.x where they apply); this branch stays as is for review.