Adopt the converged zingolib mixnet surface (ADR 0024 phase 2)

[zancas]

Follows zingolabs/zingolib#2578 (phase 1). ADR 0024 lives on the zingolib branch zingo-cli-over-nym; a working handoff sits at this repo's root as zingo-mobile-mixnet-convergence-claude-handoff.md.

Deletions (the app-side policy engine and every mint):

• MixnetCoordinator (poll loops, consent bit, publication sequencing) — replaced by subscribing to the zingolib driver's status.
• mixnet_mode_string in rust/lib/src/lib.rs and the TS RPCMixnetModeEnum re-declaration — the wire contract arrives from zingolib.
• describeRejection and the sendFailureTransform.ts substring markers ('Error: mixnet:', 'Nym mixnet') — consumers match typed variants, never prose.
• The dead legacy nym boolean: persisted setting, {false && ...} chrome in Settings/SyncStatusBar/Confirm, ButtonTypeEnum.Nym, and the settings.nym-* / send.nym-* translation keys in all five locales.

Upgrades:

• The UDL's six string-returning mixnet functions become UniFFI enums/records/typed errors generated from zingolib's types, extending the shim's golden-wire-pin pattern (the three-language fixtures) to the wallet component.
• Drop the direct crates.io zingo-netutils dependency (rust/Cargo.toml:44) — it links a second netutils copy with no SOCKS5 capability — and consume zingolib's re-exports instead.
• Pin zingolib by git REV, not branch = "dev" (the manifest comment already claims a rev pin that is not there).
• Implement iOS attach via the existing nym-proxy-ffi UniFFI framework; the iOS fail-open gate is iOS send gate fails open: a null mixnet view unblocks send on a platform with no mixnet #1235 and should be fixed first, independently.