feat(zcash_local_net)!: prefetch block templates via getblocktemplate long polling

[zancas]

What this changes

Every generated regtest block pays about 2.2 seconds inside zebrad's getblocktemplate handler. The handler builds a fresh shielded coinbase transaction for each template, and the build includes a proof. Zebra already owns the remedy. A getblocktemplate call that presents the previous template's longpollid makes zebrad precompute the next coinbase while it waits for the tip to change. Zebrad then answers the tip change with a provisional empty template built from that precomputation.

Zebrad::generate_blocks now parks that long-poll request after each mined block. Zebrad performs the coinbase proof work while the calling test syncs wallets and runs assertions. The next generate_blocks call consumes the prefetched template instead of paying the template construction inline.

Correctness guards

The prefetched template is an empty provisional block. The harness consumes it only when three conditions hold:

1. The mempool is empty.
2. The template's height equals the target height.
3. The template carries no transactions.

Any other state aborts the prefetch and falls back to the previous fetch-fresh path. The documented contract holds: generated blocks confirm the transactions the server has received.

Measured effect

Measured on zingolib's send_shield_cycle fixture, with per-phase instrumentation, two runs per configuration:

Configuration	Mining total	Empty separation blocks
Before	27.7–28.9 s	2.4–2.7 s each
After	22.3–24.0 s	0.4–1.6 s each

Transaction-bearing confirmation blocks keep the full path and their cost is unchanged.

API surface

BlockTemplate gains a long_poll_id field (deserialized from longpollid), which is the breaking surface and carries a CHANGELOG entry. submit_template_block splits into fetch_block_template, fetch_block_template_long_poll, and submit_block_from_template. mempool_txids exposes the consumption gate's mempool probe. Existing callers of submit_template_block are unaffected.

🤖 Generated with Claude Code

[zancas]

Self approved.