test(program): end-to-end chunked-STARK SBF integration test (#76)

Validates the full chunked STARK verification flow on the real
solana-program-test VM, across separate transactions -- the capstone
for #76's "largest Phase-3 piece".

tests/chunked_stark.rs:

  chunked_stark_verifies_across_transactions
    - tx1: InitializeSession (FriStark) + AppendChunk(s) uploading a
      valid 4-query depth-zero proof + BeginStarkVerify (setup gate,
      records num_queries=4)
    - asserts the session persists with cursor 0/4, setup_done, finalized
    - tx2: StarkVerifyStep batch=2 -> cursor 2/4, still open
    - tx3: StarkVerifyStep batch=5 (clamps to 4) -> complete -> the
      session PDA is closed (rent refunded)
    The cursor advancing 0 -> 2 -> 4 across three transactions, and the
    PDA closing on completion, is the on-chain proof that the resumable
    verifier + checkpoint cursor compose across transaction boundaries.

  stark_step_before_begin_is_rejected
    A StarkVerifyStep before BeginStarkVerify fails with
    StarkVerifyNotStarted (Custom(0x38)).

Observed per-instruction CU on the depth-zero scaffold: begin ~14.6K,
each step ~10-12K -- trivially under the 1.4M cap. A production
~7.8M-CU proof splits into batches each well under the cap, exactly as
designed.

Both tests pass on the VM (borsh 1.5.7 / platform-tools v1.52). Full
SBF regression: verify_proof_sbf 17 + cross_validator_determinism 4 +
chunked_handlers 8 + chunked_stark 2 = 31 SBF integration tests green.

CI: ci.yml test-sbf job now runs all four SBF suites (added
chunked_handlers + chunked_stark) so the on-chain chunked path runs on
every push.

#76 chunked execution is now implemented + validated end-to-end. The
remaining #76 work is the real Plonky3/Winterfell reference fixture
(the prover-fixture track) + the SDK auto-chunk helper.

Author: kh0ra

.github/workflows/ci.yml

@@ -128,6 +128,8 @@ jobs:
         run: |
           cargo test -p mosaic-program --test verify_proof_sbf
           cargo test -p mosaic-program --test cross_validator_determinism
+          cargo test -p mosaic-program --test chunked_handlers
+          cargo test -p mosaic-program --test chunked_stark
 
   lock-guard:
     name: lockfile guard (SBF-critical pins)

CHANGELOG.md

@@ -33,7 +33,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     (0x15) and `StarkVerifyStep` (0x16), that finalize the session, run
     the verifier setup / a query-range batch, drive the session cursor,
     and close the session (refunding rent) when the final query passes.
-    Compiles to SBF; the 25 existing SBF integration tests still pass.
+  - End-to-end SBF integration test (`tests/chunked_stark.rs`):
+    uploads a valid 4-query STARK proof, then verifies it across THREE
+    separate transactions (init+upload+begin, then step 0..2, then
+    step 2..4). The session cursor persists across transactions
+    (0 -> 2 -> 4) and completion closes the PDA -- proving the resumable
+    verifier + checkpoint cursor compose correctly across transaction
+    boundaries on the real VM, which is the entire point of chunked
+    execution. A second test asserts a step before begin is rejected
+    with `StarkVerifyNotStarted`. Both pass; the 25 pre-existing SBF
+    tests still pass (27 total SBF integration tests now).
 
 - Security disclosure contact corrected repo-wide from
   `security@wienerlabs.com` to the actually-monitored