Bump Rust toolchain to 1.98.1 (lance 12 blocked on arrow/DF migration)

[AdaWorldAPI]

This PR moves ladybug-rs to Rust 1.98.1 as part of the workspace-wide bump. What changes:

• rust-toolchain.toml is now 1.98.1.
• rust-version is 1.98.1 in the root crate and in ladybug-contract.
• All 6 Dockerfiles build from rust:1.98.1-slim-bookworm.
• The ci-master.yml and proof.yml toolchains are 1.98.1.

lance is NOT bumped to 12 in this PR. lance 12 needs arrow ^58 and datafusion ^54. ladybug-rs is on arrow 57 and datafusion 51, so pinning lance 12 alone would leave the dependency graph unresolvable. Moving arrow and datafusion is a separate migration and needs its own decision. I left a comment at the pin in Cargo.toml explaining this.

I did not build this repo locally. Its path dependencies need sibling checkouts; see this repo's CLAUDE.md §1.

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https://claude.ai/code/session_019HnekoM1EidTwQLS3oFVFm

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[AdaWorldAPI]

The failing Build and Compile Check jobs are not caused by this PR. Both fail with Could not find protoc while compiling a build script. The runner has no protobuf-compiler installed. The same two jobs, plus build, CI Summary and Proof Summary, are red on main's head with the identical failure.

This PR changes only toolchain pins (rust-toolchain, rust-version, Dockerfile tags, the CI toolchain input), not dependencies. No fix exists yet. The proposed patch is an apt-get install -y protobuf-compiler libprotobuf-dev step before the cargo steps in each affected workflow. I've left that out of this PR so it stays a toolchain-only change, and it belongs in its own PR.

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