test(dash): S8 job-notify round-trip KAT — merkle branch binds coinbase to canonical root

[frstrtr]

DASH S8 stratum-binding suite — job-notify round-trip KAT (final leaf)

Closes the S8 stratum-binding contract suite. The two prior leaves pin the
get_work()->job half:

• coinbase nonce64 slot geometry / extranonce2 width (test_dash_stratum_binding, test(dash-s8): stratum-binding contract harness — job target -> wire difficulty #629/test(dash): S8 stratum-binding contract KAT (extranonce2 nonce64-slot) #630)
• job-TARGET arithmetic (test_dash_work_job_targets, on master)

This leaf pins the remaining notify->submit seam: the mining.notify
merkle branch — computed once per job — must fold the miner's reassembled
coinb1 || extranonce2 || coinb2 back to the same canonical merkle root
the pool derives over the full tx list, for every extranonce2. That
equality is the whole reason a miner iterates the slot locally without
re-fetching the job; if the branch did not bind, every submit would carry a
root the pool rejects.

What it pins

1. Branch depth for a 4-leaf tree (two siblings, one per level).
2. Round-trip identity: zero extranonce2 → miner walk reproduces the pool root byte-for-byte.
3. Round-trip under iteration: the same branch binds distinct extranonce2 values back to their matching canonical roots.
4. Root injectivity: distinct extranonce2 → distinct block merkle root (search-space partition).
5. Wire form: merkle branch is LE internal-bytes hex (stratum.py convention), not the reversed display form.
6. Golden merkle-root byte-parity anchors.

Non-circular guard

Every expected value is derived from an independent Python sha256d merkle
walk, not from the SUT. The leaf0 anchors match test_dash_stratum_binding
exactly, confirming a shared coinbase layout. The pool-side canonical root is
computed in-test with a standard merkle reduction, separate from
merkle_branches_raw, so the round-trip equality is a real cross-check rather
than a tautology.

Oracle

frstrtr/p2pool-dash @9a0a609 — dash/stratum.py (LE merkle-branch packing) +
p2pool/work.py:421,436-437 (coinb1/coinb2 split).

Scope / safety

Pure, socket-free, node-free synthetic layout — no dashd, no live sharechain,
no template build. DASH-tree-fenced: adds test/test_dash_job_notify_roundtrip.cpp
plus its test/CMakeLists.txt registration and the two build.yml allowlist
entries only. Non-consensus, additive, GO-independent. Merge-gated on full
green — no self-merge.