compactblock: import BIP152-style compact block relay from upstream 1.2.23

Brings src/compactblock.{cpp,h} and test_compactblock.cpp across verbatim
from the upstream-v1.2.23 tag, and gives them a build.

1.2.23 is not a newer release of the line this fork ships. It is a
lightweight tag (7ab5a8c) cut 2026-07-30, on a lineage that upstream/main
(e0be70e, 2026-08-20) does not contain and no remote branch carries -- 272
commits that are not ours, against 376 of ours that are not theirs, from a
merge-base at 56df983. Merging that tag would have deleted 18 files from
src/, selftest.{cpp,h} and the whole 14-test suite among them, so the
content is ported file by file instead.

Compact block relay is the piece worth having: a peer that advertises
sendcmpct gets 48-bit keyed short IDs per transaction rather than the
transaction bytes, and reconstructs the block from its own mempool. It is
purely additive on the wire -- a peer that never sends sendcmpct keeps
receiving full block messages, so no coordinated upgrade is needed.

This commit is the primitives and their unit test only. Nothing calls
BuildCompactBlock or TryReconstructBlock yet: the P2P message handlers
(sendcmpct/cmpctblock/getblocktxn/blocktxn) live in 1.2.23's main.cpp and
are deliberately left for a follow-up, so that no attacker-controlled bytes
reach this code until that wiring is reviewed on its own.

The files needed no API adaptation -- mapTransactions, BEGIN/END and
IMPLEMENT_SERIALIZE are unchanged across the divergence, and it compiled
clean first time. Upstream never wired test_compactblock.cpp into its own
Makefile, so it had never been built there; it links standalone the way
test_bip32 does, supplying its own mapTransactions and GetAppDir to keep
main.cpp out of the link. TEST_LIBS is overridable for the same reason
NODE_LIBS is: the pinned dep tree ships libsodium as a static archive only.

All 17 checks in test_compactblock pass, all 14 selftests still pass, and
the consensus constants are unchanged against e4f898c.

1 files changed, 1 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index b18859f..77b1660 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,6 +8,7 @@ bitflash-node
 src/bitflash-node
 src/obj-node/
 src/test_bip32
+src/test_compactblock
 
 # Release artefacts (produced by 'make windows' / 'make linux')
 Bitflash-*-windows/