Persist the SNARK test caches across CI runs #3467

Why

Neither the keygen cache nor the SRS is persisted across CI runs (the toolchain-and-cache action only saves dependency artifacts through Swatinem/rust-cache), so every nightly and every slow PR leg starts cold.

What

Add an actions/cache layer for target/mithril-circuit-test-cache with a three-layer invalidation design: content-keyed directories for semantic staleness, a versions-scoped cache key for serialization format drift, and self-healing readers as backstop.

How

Paths are relative to mithril-stm/src/ unless they start with .github/.

• Add an actions/cache/restore step for target/mithril-circuit-test-cache in the test job of .github/workflows/test-rust.yml (next to the toolchain-and-cache composite step), with an exact key folding a manual generation token and a hash of the midnight-* entries of Cargo.lock
• Scope every restore-keys prefix to that versions hash so a midnight-proofs upgrade (serialization format drift) can never restore stale bytes
• Save with actions/cache/save only from the runs that execute the full slow suite (the include-slow-tests: true invocation from nightly-dispatcher.yml); PR runs stay restore-only
• Make the readers self-healing: on deserialization failure, KeyProvider::cache_state and read_optional (circuits/key_provider.rs) and TrustedSetupProvider::get_trusted_setup_parameters (circuits/trusted_setup.rs) should delete the offending file and regenerate instead of failing the test
• Prune fingerprint directories not touched by the current run before saving, so retired fingerprints do not accumulate in the archive
• Make one recursive verifying-key cache canonical for both the circuit tests and IvcSnarkProverSetup::build_for_test_degree, with the prover path storing only the additional proving key: today proof_system/ivc_halo2_snark/prover_setup.rs and circuits/halo2_ivc/tests/common/generators/setup.rs hold the same key under two protocols, because KeyProvider::verification_key treats a missing proving key as a miss and regenerates the pair
• Assess the gain (expected: removes the cold start from warm CI runs) and report it in the issue

Depends on #3433.