audit: harden the verify sidecar + add tests, benchmark, coverage docs

[dkijania]

Stacked on #2 (the HTTP verify sidecar). Review/merge #2 first; this branch's base
will retarget to main automatically once #2 merges.

An audit pass over the sidecar plus the test/benchmark coverage the workspace was
missing (it had a single test before this).

Audit → robustness fixes

The sidecar accepts an untrusted POST body, so two hardening fixes:

• Body-size cap (MAX_BODY_BYTES = 32 MiB). The body was read with an unbounded
  read_to_end into a Vec; a single oversized POST could exhaust memory. Oversized
  bodies now truncate into a clean 400 instead.
• Per-request panic isolation. Verification runs inside catch_unwind, so a
  malformed-but-decodable block that trips an assertion deep in the verifier fails
  that one request (500) instead of killing a worker thread (which would silently
  shrink the pool over time).
• Refactored main.rs → lib + thin bin so the router is a pure, unit-testable
  dispatch(method, path, body) -> (status, json).

Non-blocking observation (not changed): header_from_precomputed labels block-decode
failures as VerifierError::InvalidIndexJson (a clearer BlockDecode variant exists);
it maps to 400 correctly, so it's cosmetic.

Tests

• mina-verify unit tests — decode error paths for header_from_precomputed (bad
  JSON, missing protocol_state/proof, non-base64 proof, bare vs {version,data}
  envelope).
• mina-verify heavy integration test (#[ignore]) — verifies a real devnet
  block end-to-end and rejects a height-tampered one (ProofInvalid).
• mina-verify-server unit tests (dispatch: health / 404 / 400 mapping) and
  acceptance tests that spawn the real binary and drive it over a dependency-free
  HTTP/1.1 client (fast: health / bad-request / not-found; heavy #[ignore]: a real
  block → valid:true with correct hashes, and a tampered one → valid:false).

Fast suite runs in CI; heavy proof tests via cargo test --workspace --release -- --ignored.

Benchmark

cargo bench -p mina-verify (a harness=false binary, no new deps) times decode +
verify: warm ~0.4 s/block (~2.25 blocks/sec/core), first verify ~2.7 s (SRS
priming), VK build ~5 s.

Fixture

tests/fixtures/devnet-528700.json — a real precomputed devnet block (height 528700)
from the public mina_network_block_data bucket, shared by the heavy tests and the
bench. It is intentionally not strictly UTF-8, so it also exercises the lossy-decode
path.

Docs

Root README gains the missing mina-verify-server / mina-verify-wasm crate rows and a
Testing / Benchmarks / Coverage section; server README documents the hardening, the
network:"custom" health label under MINA_VK_JSON, a Tests section, and the measured
throughput.

🤖 Generated with Claude Code