chore: align Rust toolchains on 1.98.1

[Dzejkop]

Pin WalletKit's development, CI, crate release, and Swift release compilers to Rust 1.98.1 for the shared Rust rollout. Refresh only the Nix Rust overlay so the host, WASM, and Android shells resolve this release. Preserve the declared Rust 1.91 workspace MSRV.

Retain the compatibility fixes from the original 1.97.1 upgrade and use fixed-size chunks when decoding cached issuer schema IDs to satisfy Rust 1.98 Clippy without changing the byte format. Cargo dependencies and Cargo.lock are unchanged.

Validation

• cargo +1.98.1 fmt --all -- --check
• In the repository Nix environment: cargo clippy --workspace --all-targets --all-features -- -D warnings
• git diff --check
• A targeted activity-cache test build reached project compilation but was blocked by repeated GitHub 504 responses while fetching pinned World ID circuit artifacts; leave the test suite, cross-target builds, foreign bindings, and nightly docs to CI.

Cargo reports the existing future-incompatibility warning in proc-macro-error2 2.0.1.

Rollout dependency

Keep this PR in draft until Infrastructure #48339, Bedrock #437, Oxide #1190, Biometric Engines #648, and Flamingo #104 are ready. Version alignment will intentionally fail while the organization variable remains 1.94.1. No live variable or infrastructure change is made here.

AI usage/prompt(s)

Codex: “Adjust the coordinated Rust upgrade PRs to align on Rust 1.98.1, validate compatibility, and include Flamingo now that it participates in organization version alignment.”