refactor(stm): Reuse the cached unsafe SRS in the SNARK test setups

[damrobi]

Content

This PR includes an update to the way we use the unsafe SNARK setup in tests in order to reduce the runtime of the tests.

Changes

• Reuse existing unsafe SRS files on disk instead of regenerating them each run.
• Stop unconditionally downsizing test SRS when it already matches the circuit's degree, avoiding a needless FFT.
• Split build_for_test into a fast default (exact degree) and an explicit oversized variant, for both SnarkProverSetup and IvcSnarkProverSetup.
• Drop SRS degree from the IVC key-cache fingerprint, since derived keys don't depend on it. This lets tests at different degrees share one key cache.
• Cache the unsafe SRS by degree and seed only, shared across all parameter sets instead of duplicated per test.

Gain in test time

Baseline: 1h11m47s (nightly 30/07 https://github.com/IntersectMBO/mithril/actions/runs/30506077425/job/90756011560)

After commit 1: 1h00m56s (https://github.com/IntersectMBO/mithril/actions/runs/30538872179/job/90858636858)

After commit 2: 40m50s (https://github.com/IntersectMBO/mithril/actions/runs/30552401815/job/90904114168?pr=3443)

After commit 5: 34m49s (https://github.com/IntersectMBO/mithril/actions/runs/30643293824/job/91198342959?pr=3443)

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

Comments

Issue(s)

Closes #3433

[github-actions]

Test Results

     5 files  ± 0     209 suites  ±0   2h 26m 5s ⏱️ + 1h 31m 0s
 3 411 tests +81   3 411 ✅ +81  0 💤 ±0  0 ❌ ±0 
11 180 runs  +90  11 180 ✅ +90  0 💤 ±0  0 ❌ ±0 

Results for commit bd12ad0. ± Comparison against base commit a2e77d8.

♻️ This comment has been updated with latest results.