Add MSRV (Minimum Supported Rust Version) pinning to 1.78 for
pdftract-core and pdftract-cli. The MSRV gate prevents silent
absorption of newer Rust features that would break downstream
consumers on older toolchains.
Changes:
- CI: Add quality-matrix DAG with msrv-check step (rust:1.78-slim)
- CI: Add clippy-check, fmt-check, cargo-audit, cargo-deny templates
- README: Add MSRV badge (shields.io)
- clippy.toml: Enable msrv=1.78 for MSRV-aware lints
- CONTRIBUTING.md: Document MSRV bump policy (MINOR version event)
The rust-version was already declared in workspace Cargo.toml;
this bead adds the CI enforcement and documentation.
Refs: pdftract-2w02
