…structor slice 7/7 — FINAL)

Un-stub tracker.m_on_block_found in src/c2pool/main_btc.cpp: a share that
clears the PARENT target now reconstructs the full parent block from live
tracker state + the captured template txs and fires the connect-authoritative
dual path (P2P relay + submitblock RPC). Until now make_on_block_found (#744)
was defined but NEVER assigned, so won shares were silently dropped.

Wiring (composes the merged slices 1-6):
- TemplateCapture (#837) declared at main scope; create_share_fn seeds it,
  keyed by the freshly-minted share_hash, with the template non-coinbase tx
  set ([{"data": <raw-tx-hex>}, ...], template order) so a won share replays
  the EXACT body it committed to; a capture MISS -> valid coinbase-only block.
- make_reconstruct_closure (#839) bound to the live tracker:
    * share_fields_fn  -> share m_min_header + compile-time version +
      m_merkle_link + optional segwit txid_merkle_link (read chain DIRECTLY:
      fires on the compute thread already holding m_tracker_mutex exclusively,
      so it must NOT take read_tracker()).
    * gentx_bytes_fn   -> generate_share_transaction SSOT with a new opt-in
      out-param exposing the non-witness gentx bytes (the SAME bytes hashed to
      the committed txid; v36_active=false is byte-identical to the verify-path
      call, share_check.hpp:1807) -> merkle-consistent.
    * template_other_txs_fn -> make_template_other_txs_fn(capture.provider())
      (#840 decode contract; malformed => fail CLOSED).
- make_on_block_found (#744) dual-path: both network sinks are single-thread-
  confined to the io_context, so each io::post its write onto ioc; the audit
  reflects arm availability (has_p2p/has_rpc) while the daemon verdict is logged
  by submit_block_hex (#752) + pending_submits on the io thread.

New seam: Node::submit_block_hex_str(hex) forwards the reconstructor hex
straight to the submitblock RPC (no bytes round-trip).

btc-tree only; no CMakeLists change (all edits to already-compiled files).
btc_share_test 106/106 (reconstruct + wiring suites green).