ci: enforce declared Rust 1.85 MSRV

[ShmalexM]

Summary

• lock comfy-table to 7.1.4, which compiles on the Rust version declared by this repository
• add a dedicated Rust 1.85 CI job running cargo check --locked

Why

The current lockfile resolves comfy-table 7.2.2, which uses let-chain syntax unavailable on Rust 1.85. That makes the repository's declared MSRV fail before Flowprep itself is compiled.

Validation

Using Rust 1.85.1:

• reproduced the current main failure with comfy-table 7.2.2
• cargo fmt --check passed
• cargo check --locked passed
• all 53 tests passed
• Clippy passed with warnings denied
• release build and cargo package --locked passed

This is an MSRV/CI-only change with no product, customer, or deployment behavior. Existing PRs #2 and #3 also edit the CI workflow and may require ordinary reconciliation when rebased.