Done. src/SwapObservationHook.sol is the only file changed.

What it does

Write path — afterSwap only (the single permission it declares; afterSwapReturnDelta stays false, so it claims no part of any swap):

(, int24 tick,,) = poolManager.getSlot0(poolId);        // the pool's own post-swap tick
_observations[poolId][count % CAPACITY] = Observation({tick: tick, blockNumber: uint48(block.number)});
_observationCount[poolId] = count + 1;

CAPACITY = 16 is a constant used as the length of a storage array — it cannot be changed after deployment by anyone. No owner, no admin, no fee, no configuration; it inherits BaseHook and nothing else.

Read path — three views, all keyed by PoolKey:

• latestObservation(key) — newest entry; reverts NoObservations() on a pool never swapped on. Tick 0 is a valid tick (1:1 price), so a zeroed struct would be indistinguishable from a real entry — that reasoning is in the error's NatSpec.
• observations(key) — surviving entries oldest-first, min(count, CAPACITY) long; an empty array before the first swap. The seam is resolved by copying from count - length, so callers never see ring internals.
• observationCount(key) — all-time swaps for that pool, which keeps rising past CAPACITY. This is what makes wraparound observable; it's explicitly documented as not an index.

Invariants the tester will look for are stated in the contract: one entry per swap, not per block and not per price change (no coalescing, no dedup — a sub-tick swap re-appends the same tick); pools are fully isolated by PoolId; and it is explicitly documented as not safe as a price feed, since CAPACITY swaps in one block can flush the whole buffer.

int24 tick + uint48 blockNumber share one slot; uint48 overflows past block 2.8e14, noted in NatSpec as not worth defending in code rather than left silent.

Verification

• forge clean && forge build — clean, 94 files, no warnings.
• forge test on the untouched repo (scratch removed) — 8/8 existing tests pass.
• Scratch suite (deleted, not submitted): 13 tests via BaseHookTest with hookArtifact() overridden — the real launch pool, no mocked manager. Covers reads before any swap (revert + empty array + unknown-pool reads that aren't stale), three swaps in one vm.roll'd block, exact fill to capacity, wraparound at CAPACITY+5 and 3*CAPACITY+1 with order checked across the seam, two live ETH/token pools not seeing each other, and one pool wrapping twice without evicting the other's single entry. Plus the starter's own 8 hook-agnostic tests re-run against this hook. 29/29 pass.
• forge fmt --check clean.

No build-config change was needed, and no dependency was added — everything used (StateLibrary, PoolIdLibrary) is already vendored under lib/. The file is left uncommitted in the working tree; I didn't create a commit since you didn't ask for one.

Done. test/SwapObservationHook.t.sol is the only file added; forge build is clean (no compiler or lint warnings) and forge test passes 39/39 (8 pre-existing + 31 new).

What the suite covers

Built on BaseHookTest, so every observation comes from a real PoolManager calling the hook on a real launch pool — nothing mocked.

Reads before anything happened — untraded pool reports count 0 and an empty buffer; latestObservation reverts NoObservations (tick 0 is a real price, so a zeroed struct would be a lie); a pool key that was never initialized reads the same way; a reverted swap (SwapAmountCannotBeZero) records nothing; liquidity add/remove and donates record nothing.

What one entry is — post-swap tick and block, cross-checked against getSlot0. The sharp one: a swap stopped early by a price limit fills 3 of 100 ETH offered, and the recorded tick is the limit tick — a value no implementation deriving the tick from the amount or the delta could produce. Also sells and exact-output swaps, and negative ticks via a second pool started at −60000 (the packed int24/uint48 slot is where a sign-extension bug would hide, and the launch pool never goes negative).

Several swaps in one block — four buys, no vm.roll: four entries, same block number, execution order preserved, nothing coalesced. Plus three sub-tick buys that provably don't move the tick and still append three identical entries.

Wraparound — exactly-to-capacity; one past capacity (whole buffer compared, oldest confirmed gone, newest confirmed last not first); 53 swaps = three laps plus five, so the seam sits mid-array; latest == last ordered entry at every step; strict monotonicity across the seam; and the emitted slot index decoded from logs, wrapping 15 → 0.

Two pools — neither sees the other's entries in either direction; wrapping a busy pool twice leaves a quiet pool's single entry untouched; and two pools differing only by fee tier stay separate.

No fee, no role — a non-manager afterSwap call reverts and writes nothing (observations can't be forged); unrequested callbacks revert HookNotImplemented even for the manager; permissions are afterSwap and nothing else, afterSwapReturnDelta off; hook holds no ETH, no token, no ERC-6909 claim; plain ETH transfers are rejected; probes for owner()/admin()/transferOwnership/setCapacity/sweep all fail.

Plus a bounded fuzz over 0–22 swaps asserting the buffer is always the last min(count, CAPACITY) in order.

Defects

None found — no .imd-findings.json written. I read the implementation adversarially and every path I could reach behaved as its documentation claims. To confirm the suite isn't just agreeing with whatever the code does, I ran it in scratch against five deliberately broken copies of the hook, each killed by multiple tests: observations() ignoring the seam (5 failures), latestObservation off by one (11), per-block coalescing (12), a single shared buffer instead of per-pool (3), and a ring that stops wrapping (5).

One thing worth flagging as a judgment call rather than a finding: uint48(block.number) truncates silently past block 2.8e14. The contract documents it and the reasoning holds (~100M years at 12s blocks), so I tested that type(uint48).max round-trips exactly and did not write a test blessing the wrapped value.