chore(deps): upgrade Zcash stack for NU6.3

[gustavovalverde]

Why

Z3's defaults currently combine a Zebra 6.0 release candidate, a Zaino release that cannot consume Zebra 6's Ironwood-era RPC responses, and an older Zallet alpha. That set cannot exercise the current NU6.3 stack as one supported unit.

This upgrades the three services together while preserving deliberate, reproducible image pins. It supersedes the moving Zebra tag proposed in #47 with a tested release combination.

Service upgrades

Service	Previous	New
Zebra	6.0.0-rc.0	6.2.0
Zaino	0.5.1-no-tls	0.6.0-no-tls
Zallet	0.1.0-alpha.4	0.1.0-beta.1

The matching source-build refs, image override examples, Zallet feature gates, and operator documentation move with those defaults. Regtest now activates every supported upgrade through NU6.3 at height 2, with Zebra and Zallet using the same schedule and Zaino discovering it from Zebra.

The rebuilt Zallet beta image contains all three binaries on both published architectures, but omits the system CA bundle required by its Zaino backend. The Compose prerequisite sidecar now copies Alpine's CA bundle into an internal volume and mounts it read-only into Zallet. This keeps the upstream image immutable and avoids a host-specific certificate bind mount.

Upgrade notes

• Existing disposable regtest chain and wallet volumes use the previous activation schedule. Reset them and rerun scripts/regtest-init.sh before using this version.
• Zaino 0.6 migrates its finalized-state database from schema 1.2 to 1.3. Back up persistent Zaino data before upgrading if rollback to 0.5.1 must remain possible.
• The Zallet image stays pinned by digest because the beta tag was rebuilt after release to correct its multi-architecture binary packaging.