tools(btc): pre-stage full G3b standup — dual-path broadcaster, one-command unblock

[frstrtr]

Pre-stages the full BTC G3b standup so the eventual unblock (testnet3 IBD completes / SHA256d rig key #387/#388 lands) is one command, not a fresh scramble. Authoring + dry-run need no rig key and no IBD-complete. Scripts-only, fenced under tools/testnet/, no core/consensus change.

Stacks on the VM130 readiness gate (base = btc/vm130-readiness-gate).

What lands

• vm130_btc_readiness_gate.sh — EXTENDED with dual-path broadcaster preconditions on top of the existing IBD/template gate:
  • ARM B: submitblock RPC method is live (RPC fallback path).
  • ARM A: getconnectioncount >= 1 so an on_block_found P2P relay has a landing site (a 0-peer node would accept submitblock but silently drop a relay → ARM A would pass falsely). Fails closed.
• vm130_btc_standup.sh — one-command orchestrator: composes the gate then launches c2pool-btc. single or tuned (2-instance A→B crossing via opt-in --sharechain-port btc: --sharechain-port flag for isolated second instance (G3b tuned-net) #457). --dry-run prints the full plan and runs nothing.
• g3b_block_acceptance.py — runnable G3b FOUND→ASSEMBLED→ACCEPTED→BROADCAST over both arms × 3 regimes (v35/HYBRID/v36). --dry-run asserts reaches_network is orthogonal to (arm, regime); --live drives ARM B submitblock for real and marks ARM A rig-gated (never silently passed).

Dry-run output (this PR head)

== G3b dry-run model: FOUND->ASSEMBLED->ACCEPTED->BROADCAST (both arms x 3 regimes) ==
  arm=A_p2p_relay    regime=v35     => ACCEPTED
  arm=A_p2p_relay    regime=HYBRID  => ACCEPTED
  arm=A_p2p_relay    regime=v36     => ACCEPTED
  arm=B_submitblock  regime=v35     => ACCEPTED
  arm=B_submitblock  regime=HYBRID  => ACCEPTED
  arm=B_submitblock  regime=v36     => ACCEPTED
ALL PASS: all_reach=True arm_regime_orthogonal=True

Standup --dry-run (single + tuned) plans print and run nothing; bash -n + py_compile clean.

I do not self-merge — operator/integrator tap.