…nstruct closure (#279)

The won-block reconstruct closure the run-loop installs as
ShareTracker::m_on_block_found is today an interim nullopt stub. The faithful
path (reconstruct_won_block_from_template) needs five inputs; two of them the
won share carries verbatim and version-agnostically: small_header
(m_min_header) and merkle_link (m_merkle_link).

Add dgb::coin::won_share_inputs(share) -> {small_header, merkle_link}: a pure
read of already-validated share_info fields (no PoW / coinbase commitment /
share format / PPLNS touched; p2pool-merged-v36 surface NONE). Duck-typed on
the share to stay off share.hpp tracker/base_uint TU (the limited-TU build
trap). Binds to the live dgb::Share in the run-loop once the remaining inputs
(gentx/gentx_hash inverse-of-out_gentx, template_other_txs retention) land.

+2 KATs into dgb_reconstruct_won_block_test (existing target, no allowlist
churn): verbatim extraction, and seam-output drives a byte-identical block to
passing the fields directly. 9/9 PASS. Fenced to src/impl/dgb/.

Co-authored-by: frstrtr <frstrtr@users.noreply.github.com>

…osure builder

Compose the run-loop WonBlockReconstructor from the captured-GBT template
source (#271) instead of the share tx_hash_refs: won_share_inputs (#279,
small_header + merkle_link) ++ unpack_gentx_coinbase(gentx) ++
template_other_txs -> reconstruct_won_block_from_template. Version-agnostic
(dissolves the v34+ no-m_tx_info concern) and fail-closed (any error ->
nullopt, announce + audit, RPC submitblock fallback still attempts).

Seam-injected so it is unit-tested with no live ShareTracker/mempool. +5
KAT into dgb_reconstruct_closure_test (existing target, no new allowlist
entry): success==reconstruct_won_block_from_template, empty-template=>valid
coinbase-only (not nullopt), unknown-share / malformed-gentx / template-throw
each fail closed. 11/11 PASS. src/impl/dgb/ only; p2pool-merged-v36 surface
NONE. This is the builder the main_dgb run-loop installs in place of the
nullopt stub (next slice).

… nullopt stub)

main_dgb.cpp --run binds tracker().m_on_block_found to the FAITHFUL
make_reconstruct_closure_from_template (#280) in place of the interim
nullopt stub. The three version-agnostic won-block inputs are bound to
the live sharechain tracker:
  - won_share_fields_fn: share.m_min_header + m_merkle_link (#279)
  - gentx_bytes_fn: generate_share_transaction(...).GentxCoinbase.bytes
    (#173 SSOT); v36_active re-derived from the share compile-time
    version inside GST, byte-identical to the verify-path invocation
  - template_other_txs_fn: captured-GBT non-coinbase set (#271), empty
    today (no per-job template retention + unfed embedded mempool =>
    coinbase-only), correct-and-empty, NOT fail-closed

Fires on the compute thread already holding the tracker lock, so the
fns read tracker().chain directly (no read_tracker(), no self-deadlock).
Fail-closed end to end: any builder error -> nullopt (announce + audit;
RPC submitblock fallback still fires). Fenced to main_dgb.cpp; build
EXIT=0; selftest OK. #82 OPEN pending a live UpdateTip won-block test.