chore: declare MSRV (1.88) and verify in CI

[scarmuega]

Summary

• Declare workspace MSRV as Rust 1.88 via [workspace.package].rust-version, inherited by all 22 member crates with rust-version.workspace = true.
• Add a msrv CI job pinned to dtolnay/rust-toolchain@1.88 that runs cargo check --workspace --locked --all-targets so any regression — code or transitive dep update — breaks CI.
• Document the MSRV and bump policy in README.md.

Why 1.88

• pallas-network2 uses edition 2024 → hard floor of 1.85.
• The locked dependency graph resolves serde_with 3.19.0 and darling 0.23.0, both of which require rustc 1.88. That's the real-world floor today.
• 1.88 was released 2025-06-26 (~11 months old), well within a "stable − 3 releases" window.

Verified locally with cargo +1.88 check --workspace --locked --all-targets — clean.

Test plan

• CI MSRV (1.88) job passes
• Existing Test, Clippy, Docs, Features jobs continue to pass

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 5cd5004d-9169-4526-a00c-6bec2ad2f0b7

📥 Commits

Reviewing files that changed from the base of the PR and between 7beb8a8 and 5e4b90e.

📒 Files selected for processing (25)

• .github/workflows/ci.yml
• Cargo.toml
• README.md
• examples/block-decode/Cargo.toml
• examples/block-download/Cargo.toml
• examples/crawler/Cargo.toml
• examples/n2c-miniprotocols/Cargo.toml
• examples/n2n-miniprotocols/Cargo.toml
• examples/p2p-discovery/Cargo.toml
• examples/p2p-initiator/Cargo.toml
• examples/p2p-responder/Cargo.toml
• pallas-addresses/Cargo.toml
• pallas-codec/Cargo.toml
• pallas-configs/Cargo.toml
• pallas-crypto/Cargo.toml
• pallas-hardano/Cargo.toml
• pallas-math/Cargo.toml
• pallas-network/Cargo.toml
• pallas-network2/Cargo.toml
• pallas-primitives/Cargo.toml
• pallas-traverse/Cargo.toml
• pallas-txbuilder/Cargo.toml
• pallas-utxorpc/Cargo.toml
• pallas-validate/Cargo.toml
• pallas/Cargo.toml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/declare-msrv

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