feat(fast-inbox): same-block L1-to-L2 message consumption for non-first blocks (A-1432)

[spalladino]

What

Makes every block's BlockConstantData.l1_to_l2_tree_snapshot the post-bundle snapshot for that block — first block or not — so a public/AVM tx in block N can read the L1-to-L2 messages block N inserts (same-block consumption), instead of only from block N+1.

Previously the non-first tx-carrying variants (block_root, block_root_single_tx) read their start snapshot from constants.l1_to_l2_tree_snapshot and never asserted the post-bundle root, giving next-block visibility for mid-checkpoint insertions. Now they:

• take the start L1-to-L2 snapshot as a witness input (previous_l1_to_l2), pinned by block-merge continuity (right.start_state == left.end_state) to the previous block's end state — the checkpoint root forces the leftmost block to be a first-block variant, so every non-first block has a left neighbour pinning its start;
• append their bundle and assert validate_l1_to_l2_tree_snapshot_in_constants(constants, new_l1_to_l2), exactly like the first-block variants.

The tx-less variants (block_root_empty_tx_first, block_root_msgs_only) carry no tx constants to read against and already witness their start snapshot, so they are unchanged.

Soundness

The witnessed start snapshot cannot be forged: block-merge / checkpoint-root continuity asserts right.start_state == left.end_state, and the checkpoint root asserts the leftmost block is a first-block variant (whose start is pinned to the previous checkpoint). The start is therefore anchored, and the new assert pins the tx-constants snapshot to the computed post-bundle root. The start is not derived from "constants minus the bundle" (not expressible).

Bit-identical pre-flip (public inputs)

Non-first bundles are empty today, so new_l1_to_l2 == start == today's constants value; the new assert passes with unchanged values and the block-root public inputs / state are unchanged. The circuit constraints, private-input ABI, proof, and VK do change (hence the regen below). This is a restructure-now / flip-minimal change.

Flip follow-on (not in this PR)

The recursive pinning of the witnessed previous_l1_to_l2 is already exercised by block_merge::tests::consecutive_block_rollups_tests::non_consecutive_l1_to_l2_message_tree_snapshots (a right block whose start_state.l1_to_l2_message_tree — i.e. this witness — doesn't match the left block's end is rejected on continuity).

One flip requirement this change surfaces: today the prover gives every non-first block the checkpoint's post-first-block snapshot as its previousL1ToL2 (block-proving-state.ts), which is correct only while non-first bundles are empty. Once non-first blocks insert their own bundles (A-1384), the prover must instead feed block N+1 the end snapshot of block N plus the matching frontier hint. Flagged for the flip plan (A-1384).

Tests

Adds negative nargo tests: a non-first block (two-tx and single-tx) whose constants.l1_to_l2_tree_snapshot differs from its post-bundle root must fail. Red/green verified locally — with the new asserts removed the four negative tests report "Test passed when it should have failed"; restored, they pass. rollup_lib block_root suite green (63 tests), block_merge (45), checkpoint_root structure tests (17).

TS wiring

Mirrors the new previous_l1_to_l2 field through stdlib (BlockRootRollupPrivateInputs / BlockRootSingleTxRollupPrivateInputs serialization + factory), the Noir ABI conversion (server.ts), and the prover-client block-root input builders (block-proving-state.ts), which pass the block's lastL1ToL2MessageTreeSnapshot (for non-first blocks, the checkpoint's post-first-block snapshot). stdlib builds and its serialization test passes.

Artifact regen (rides CI / mainframe)

This changes the block_root and block_root_single_tx circuit ABIs, so it invalidates their VKs, the checkpoint-root/block-root Prover.toml sample inputs, and the generated noir-protocol-circuits-types/src/types/index.ts. These are not regenerated locally (bb write_vk OOMs on large circuits on dev boxes; the generated index.ts also depends on recompiled circuit artifacts). They ride the seeded-S3-cache mainframe/CI flow, consistent with the rest of the Fast Inbox stack.

Stack

Stacked on spl/a-1427-inbox-parity; the L1 PRs (#24771, #24773) are rebased on top of this. Part of the Fast Inbox stack (umbrella #24774).

Fixes A-1432