test: stackToFlow malformed-stack revert paths (#328)

[thedavidmeister]

What

Completes test coverage for issue #328 (LibFlow.stackToFlow malformed-stack reverts). Prior work already covered the empty / one-sentinel / two-sentinel paths via testStackToFlowRevertsOnEmptyStack, testStackToFlowRevertsOnOneSentinelOnly, testStackToFlowRevertsOnTwoSentinelsOnly in test/src/concrete/Flow.preview.t.sol. This PR adds the two remaining uncovered sub-cases.

TEST-ONLY: no src/ changes.

Already covered (verified, not duplicated)

• empty stack, one sentinel, two sentinels — existing testStackToFlowRevertsOn* tests.

Added

• testStackToFlowRevertsOnMalformedTupleCount — a stack whose top section is not a multiple of the ERC20 tuple size (4). consumeSentinelTuples strides 4 * 0x20 bytes from the top, so the misaligned section makes the ERC20 pass bind to the ERC721 sentinel, the ERC721 pass bind to the ERC1155 sentinel, and the ERC1155 pass find no sentinel left. Asserts the exact MissingSentinel(RAIN_FLOW_SENTINEL) revert.
• testStackToFlowSentinelInsideTupleCorrupts — a RAIN_FLOW_SENTINEL planted in a tuple field (where the ERC20 amount would be). It is matched as the ERC20 boundary, so the genuine ERC20 sentinel is consumed by the ERC721 pass and the genuine ERC20 tuple is misread as an ERC721 tuple whose token is the truncated sentinel. This path does NOT revert; the test pins the full corrupted FlowTransferV1 (erc20.length=0, erc721.length=1 with the planted-sentinel token + shifted fields, erc1155.length=0).

Mutation matrix

Each new test was mutation-validated against the tuple-count / sentinel-position handling in src/lib/LibFlow.sol (mutations reverted after; src/ is clean in this PR).

Mutation in LibFlow.stackToFlow	RevertsOnMalformedTupleCount	SentinelInsideTupleCorrupts
ERC20 arity 4 → 3	FAIL (killed: no longer reverts)	PASS
ERC721 arity 4 → 5	PASS	FAIL (killed: now reverts MissingSentinel)

Each test is killed by a distinct arity mutation, confirming both are discriminating.

Build / test

• forge build: clean.
• forge test --match-contract FlowPreviewTest: 13 passed, 0 failed (11 pre-existing + 2 new).

Pre-existing reds on main (NOT introduced here)

• rainix-sol-static (slither) is red on main; PR fix: remove redundant statement flagged by slither (unblocks rainix-sol-static) #476 fixes it (unmerged).
• forge test on main also has one pre-existing fuzz red, testFlowBasicValidateMultipleSignedContexts in test/src/concrete/Flow.signedContext.t.sol ("next call did not revert as expected", deterministic under the fixed 0xdeadbeef seed). Confirmed to fail on a clean 772e568b checkout with this branch's change stashed, so it is unrelated to and untouched by these test additions. No issue currently tracks it.

Closes #328

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 517b856a-8a1c-4626-9605-19f38be5b7c2

📥 Commits

Reviewing files that changed from the base of the PR and between 772e568 and 12e8f0e.

📒 Files selected for processing (1)

• test/src/concrete/Flow.preview.t.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-14-issue-328

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