Ironwood migration-dummy blocks the helper send (create vs create-max) + engine-bump fix

[danielgorgonha]

The bug: Ironwood migration-dummy breaks the helper send path

Post-NU6.3, a single/multi-note spend from the Orchard pool creates a wallet-controlled
zero-value "migration dummy" spend. create_pczt_from_proposal (in the pinned engine 42ffd0d)
does not stamp the ZIP-32 derivation on that dummy, so an external signer (our FROST flow, and
even a seed wallet's pczt sign) skips it as "not ours" → MissingSpendAuthSig at extract → the
broadcast fails.

This blocks the Architecture-B / convergence broadcast for any vault funded in the Orchard pool.

Where create is used (all hit the bug for Orchard-funded vaults)

• orchestrator/src/send.rs::build_unproven → Payment: pczt::create (single output).
• same → Payroll: konclave-signer build-payroll (multi output).
• Funding a vault (seed → vault) via the CLI also hits it (seed's own Orchard spend).

create is correct semantically (send X, change back) — the problem is only the Orchard dummy.

The two paths to fix it

(1) Workaround — create-max migration (interim, Phase-14-validated on mainnet)

zcash-devtool pczt create-max --address <self> <account> spends all notes to one output with
no dummy. Pattern:

1. Migrate the vault's Orchard → Ironwood (a create-max self-send, once). Funds land in the
   Ironwood pool.
2. Payments from the Ironwood pool then work with plain create (the Ironwood pool signs its
   own dummy correctly).

create-max does not fit a normal payment (spends everything to one address, no change), so it is
only for the migration step. Needs a small helper addition (a pczt::create_max wrapper + a "migrate
first" step in the send flow), gated so it can be removed once (2) lands.

(2) Proper fix — engine bump to librustzcash ≥ 51385a15 (definitive)

Commit 51385a15 ("Stamp ZIP 32 derivation on wallet-controlled zero-value spends in PCZTs", #2777,
merged 2026-07-27) makes plain create work — no create-max. But it is a major bump:
pczt 0.7 → 0.9, 406 commits past our pin, which breaks the byte-for-byte PCZT wire compat between
konclave-signer and zcash-devtool (ADR-0002). Requires rebuilding both at the new pin +
updating the signer's extract/inject to pczt 0.9. A dedicated engine-migration task.

Plan

• This issue / branch (fix/ironwood-createmax-migration): add pczt::create_max + a migration
  step to the helper, interim, so the convergence broadcast works now on the current engine.
• Separate issue (engine bump): move to librustzcash 51385a15 / pczt 0.9, re-align ADR-0002,
  drop the workaround.

The branch is prepared with the create_max primitive so it is ready to complete/enable when we run
the money-gated broadcast.

[danielgorgonha]

Branch fix/ironwood-createmax-migration started: pczt::create_max primitive added (orchestrator/src/pczt.rs). Remaining: wire a migrate-first step into send.rs so an Orchard-funded vault self-sends (create-max) into the Ironwood pool before the payment. Ready to complete + enable for the money-gated broadcast.