release(7.15.0-rc22): Zcash NU6.3 Ironwood support

[BitHighlander]

Why

After NU6.3, shielding value into the Orchard slot is rejected by consensus because Orchard value balance must be non-negative. New shielded value must enter Ironwood in transaction v6.

What changed

• accept the new protocol pool selector and Ironwood component digest
• implement ZIP-229 five-component v6 sighashes
• implement ZIP-2005 V3 note commitments (H_rcm) for Ironwood outputs
• verify Ironwood action digests, proofs, binding signatures, output commitments, transparent data, and fees on-device
• retain the existing v5 Orchard path
• consolidate V2/V3 commitment code so the release image remains within flash

This is the 7.15.0 RC22 candidate.

Canonical dependency pins

• feat(zcash): add Ironwood signing metadata keepkey/device-protocol#113 merged to upstream branch at f2246cebea8f96fcd7ec2883588a784a60b430ae
• test(zcash): cover NU6.3 Ironwood signing keepkey/python-keepkey#200 merged to upstream branch at d88a073a5af2d83e0f1f19574665ca1a44789414
• python-keepkey and firmware both pin protocol f2246cebea8f96fcd7ec2883588a784a60b430ae
• no dependency fork SHA and no unpublished package pin

Validation

• ./scripts/build/docker/device/release.sh — passed
• full release image: 649,972 bytes of 655,360-byte ROM budget (5,388 bytes free)
• ./scripts/build/docker/emulator/debug.sh — 388/388 tests passed
• exact clang-format 20 check — passed
• Pallas API boundary check — passed
• new fixed vectors cover Ironwood V3 cmx and the v6 ZIP-229 sighash