dash(fees): W5-A fold-backed fee pricing behind a hash_serialized_2 graduation gate

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W5-A: fold-backed fee pricing behind a hash_serialized_2 graduation gate

Goal (dashd-cut Tier-2 "W5", leg A): close the DOMINANT dashd-only tx class behind the gbt-xcheck-txmerkle mismatches — 20/34 txs across the 26-event diagnosis are ordinary fee-payers spending coins OLDER than the replay horizon. The forward-built UTXOViewCache cannot price them, compute_fee_locked marks them fee-unknown, the conservative exclusion drops them, embedded serves a strict SUBSET of dashd, and the guard swaps to the dashd arm. This PR wires the W3 full-history UTXO fold (replay_utxo_fold.hpp) in as the mempool's THIRD coin source — behind a reward-safety graduation gate.

DARK / flag-gated

--embedded-utxo-fold-fees DBPATH + required --embedded-utxo-fold-expect HEX (the operator RESTATES the anchor hash — a stale/foreign DB cannot graduate on its own say-so). Default OFF: nothing is constructed, Mempool::m_fee_coin_lookup stays empty, production byte-identical. DASH lane only. No wire/consensus/share-format change.

The graduation gate (reward-safety)

A wrong fee VALUE can overstate coinbasevalue = an invalid found block = a lost block — the tx-merkle guard does not cover fee values. So fold answers are arithmetically unreachable from the fee path until ALL of:

1. GRADUATED — ReplayUtxoFold::try_graduate() runs with the cursor standing exactly on the pinned anchor (h=2,516,758) and proves hash_serialized_2 byte-equal to dashd's gettxoutsetinfo (3d14913768a9d492bfa7a42fe9b111cff625b80e35bb4133e1d60cf3991c2319). PASS writes the durable 'G' record (format v2); FAIL poisons the fold (latched refusal, nothing written). Restart restores trust ONLY from a recorded pass that also matches the operator restatement.
2. LIVE at the tip — fold cursor within 2 blocks of the last observed tip (a catching-up fold could hold since-spent coins ⇒ superset ⇒ guard trip).
3. UNDISARMED — any feed refusal (gap, store error, gate FAIL, prev-hash linkage break = the reorg tripwire, structurally preventing a silent orphan fold) latches disarm until restart. With ChainLocks live, mainnet reorgs are ~nonexistent; the latch is a rare-case backstop.

Every failure path reverts to today's fee-unknown exclusion. The #1218 gbt-xcheck-txmerkle guard is untouched and stays the referee — this PR makes the arm MATCH, it never relaxes the guard.

Pieces

• replay_utxo_fold.hpp: format v2 + 'G' graduation record, try_graduate() / graduation() / poison latch, KAT-injectable anchor, opt-in prev-hash linkage check (adapter always arms it; pre-existing synthetic-chain KATs byte-identical).
• fold_fee_source.hpp (new): FoldFeeSource — trust latch + threading adapter (fold core is single-thread by design). Lock order Mempool::m_mutex → FoldFeeSource::m_mutex; the feed path never calls into Mempool — no cycle. Missing / spent / immature-coinbase outpoints all answer false.
• mempool.hpp: set_fee_coin_lookup — a slot SEPARATE from the pinned-tx m_external_coin_lookup (the dashd-RPC arm; routing mempool pricing through it would be an unflagged production change). Consulted only on the view-miss + in-pool-parent-miss double miss, in compute_fee_locked AND in the selection-time vin resolution (pricing without resolution would be undone by the stale-input guard).
• main_dash.cpp: flag family; feeds = the W2 --replay-bulk lane (Tee side consumer; genesis start-height sentinel refuses a cold fold above h=1 at STARTUP instead of a GATE FAIL 15 h of download later; tip_exclusion=0) + the live block_connected leg (idempotent acks, tolerated gaps, note_tip).

KATs (test_dash_fold_fee_source.cpp, rides the test_dash_mempool target)

Red/green pair: UngraduatedFoldDoesNotPrice (master's exact behaviour with the fold wired and holding the coin — the red shape) vs GraduatedFoldPricesOldCoinAndSelectsIt (exact fee, SELECTED into the template). Plus SpentCoinInFoldNotPriced, ImmatureCoinbaseInFoldNotPriced, GateFailPoisonsAndDisarms, TipLagDropsTrust, GraduationRestoreAcrossReopen (wrong restatement ⇒ ignored), PrevLinkageRefusesOrphanFold.

What this PR does NOT include

The full genesis→anchor fold run to the live graduation gate (~29-31 GB download, 6-16 h on vm905) is the SUBSEQUENT soak; the PR lands on KATs + mechanism. The DSTX leg (14/34, necessary-but-not-sufficient) is the sibling PR.

Measurement plan (post-merge, canary posture: --coin-rpc kept, guards active)

1. BEFORE: 24 h on this build, both flags OFF (byte-identical dark ship) — re-baseline the mismatch rate (the 26/34 figure predates this build).
2. Fold run on vm905 to the anchor; graduation record + --replay-utxo-hash --replay-utxo-expect exit-0 as independent re-check.
3. AFTER-1: --embedded-utxo-fold-fees armed on the canary — expect the 20/34 fee-unknown class to close; classify residual dashd-only txs (should be ≈pure DSTX).
4. Acceptance (with the DSTX PR): 24 h, ZERO gbt-xcheck-txmerkle-mismatch events over all tx-carrying templates, guard still armed; quote fallback-TIME share as well as event counts.

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vm905 build + KAT evidence (BLS=REAL)

• Build: cmake -DC2POOL_DASH_BLS=ON -DDASHBLS_ROOT=~/dashbls-prefix on vm905, exit 0; [init] DASH BLS backend: REAL.
• test_dash_mempool (mempool + W3 fold + new W5-A suites): 71/71 PASSED — including FoldFeeSource.UngraduatedFoldDoesNotPrice (red shape = master behaviour), FoldFeeSource.GraduatedFoldPricesOldCoinAndSelectsIt (green), spent/immature exclusion, GateFailPoisonsAndDisarms, TipLagDropsTrust, GraduationRestoreAcrossReopen, ReplayUtxoFoldGate.PrevLinkageRefusesOrphanFold.
• test_dash_isdlock regression: 12/12 PASSED.

Fold run STARTED on vm905 (the subsequent-soak half; PR does not block on it):
~/utxo-fold/, genesis start (--replay-bulk-start 1), capture retained (~/utxo-fold/capture, one-chance rule from #1147), separate ports (9033/8199), nice/ionice — no interference with running soaks. Startup log confirms the dark posture:

[run] W5-A FOLD FEE SOURCE armed (DARK until graduated): db=/home/ubuntu/utxo-fold/db fold_h=0 graduated=no anchor_h=2516758
[run] W5-A: UTXO fold FEEDS from the bulk lane (Tee side; fold cursor h=0)

Monitor: ssh vm905 "grep -E 'GATE (PASS|FAIL)|REPLAY-UTXO|FOLD-FEE' ~/utxo-fold/fold.log | tail" — the GATE line at h=2,516,758 is the graduation event; expect a 6-16 h window (download-bound).

Sibling: #1238 (W5-B DSTX leg). Both legs are needed to close all 34/34 dashd-only txs; acceptance = 24 h ZERO tx-merkle mismatches with guards armed.