Fix standard SV2 Merkle root byte order

[feepool]

Summary

Fix standard Stratum V2 mining jobs so the advertised NewMiningJob.merkle_root
matches the Merkle root used when CKPool validates submitted shares.

Problem

The standard-SV2 path computed the raw SHA256d Merkle digest and then applied
CKPool's existing flip_32() conversion before placing it in the SV2 U256
field. SV2 carries the digest bytes in little-endian integer order, so this
word swap advertised a different Merkle root from the one used by
submission_diff().

Miners consequently hash a different header than CKPool validates. V1 is not
affected because this conversion is specific to the standard-SV2 job path.

Fix

• Preserve the raw SHA256d digest bytes for the standard-SV2 U256 Merkle-root
  field.
• Add a small serialization helper documenting the byte-order contract.
• Add a regression test using a non-symmetric digest pattern so a future
  word-swap regression is caught.

Validation

Built from current upstream master (5fca384e) with:

./configure --enable-sv2
make -j$(nproc)
make check

Configuration confirmed IPC mining and SV2 enabled. The upstream test suite
passed all 9 tests with 0 failures. The corrected binary also built
successfully with no compiler warnings from this change.