Compile the Rust quickstart as a real crate

[midenotch]

Close: #189

The unreachable! placeholder is gone. generate_stealth_address now takes viewing_seed directly — which is what callers actually have after derive_stealth_keys. The old viewing_seed_for_pub indirection that caused the panic is deleted.

MDX updated: All 6 crypto blocks lost no-check. The keypair.rs block uses the correct stellar_strkey::ed25519::PrivateKey::from_string / stellar_strkey::ed25519::PublicKey(bytes) API. The Cargo.toml block reflects the network feature split.

CI updated (snippets.yml): The check-snippets-extended job gained three new steps after the existing extended-snippets check — cargo build, cargo fmt --check, cargo test — all scoped to snippets/rust-quickstart with --features testnet and no network flag.

Tests: 6/6 pass, formatting clean.

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[truthixify]

The required snippet job fails before the new crate checks run: removing no-check makes the existing extended checker compile six dependent fragments as standalone rustc files, producing unresolved imports/types. The crate is also a manually duplicated copy rather than being built from or checked against the published fences, so the docs can drift while CI stays green. Make the existing checker pass and add deterministic extraction or a drift assertion tying the crate sources to the documented code.