add Sv2 string to coinbase tag standard

[plebhash]

as proposed in #2141 (comment)

[GitGab19]

Why did you change tests from predefined static values to real shares research?

[plebhash]

Why did you change tests from predefined static values to real shares research?

adding Sv2 to the scriptSig changes the coinbase tx, which changes the merkle root, which invalidates every hardcoded nonce in these tests, so they had to be re-mined either way.

---

when I originally wrote those pre-defined shares it was a very laborious process to find the right nonces and hardcode them, so this time I went with a bounded search instead.

it's deterministic (fixed ntime/extranonce/version), so no flakiness, just an extra bit of CPU on every execution.

and only the positive cases became searches — the DoesNotMeetTarget tests still use fixed shares, so a too-lax target check would still fail.

---

I didn't see major harm in this approach, but happy to switch back if you prefer to stick to the old way.

I'm sure agents could re-mine the constants now if I give them explicit instructions.

[plebhash]

@GitGab19 incorporated suggestions with pre-mined shares

fwiw #2236 was also introducing dynamic shares into tests, adapting accordingly before we merge

[bit-aloo]

ACK