build!: replace OpenZeppelin and murky with solady

[mfw78]

Based on develop. #62 will be restacked on top of this.

src used OpenZeppelin for exactly two things: MerkleProof.verify at a single call site, and SafeCast.toUint32 in three files. Both have direct solady equivalents, and MerkleProofLib.verify has a memory variant matching the existing call, so the swap is mechanical.

murky goes too. Its Merkle was only used by the test helper, and solady's MerkleTreeLib covers the same surface. That removes the transitive openzeppelin-contracts murky vendored, so this is a real removal rather than a partial one.

It also aligns construction with verification: ComposableCow verifies with MerkleProofLib, and MerkleTreeLib is designed to pair with it. Previously trees were built with murky and verified with solady, which happened to work.

The two are not interchangeable, which was worth checking

For leaf counts that are not powers of two they build different trees:

leaves	2	3	4	5	6	7	8	9
same root	yes	no	yes	no	no	no	yes	no

What holds either way is that a proof verifies against a root from the same library, for every count tested. That is the only property anything here relies on, since each tree is built and verified by one library and nothing mixes them. Worth stating because the naive reading of "they diverge" would be that the previous setup was broken; it was not.

Gas

Measured against the same Safe, forge-std and optimizer settings, so this isolates the dependency change.

Test	delta
setRootWithContext_e2e	-41,025
setRoot_e2e	-41,003
settle_e2e	-39,552
setSwapGuard_e2e	-11,330
payloadTree_NormativeConstruction	-7,210
createAndRemove_e2e	-5,786

95 of 128 entries move, 35 rise but none by more than 45, total -152,161 gas. Median unchanged at 0.000%. The merkle-heavy paths dominate, which is what changing the tree library should look like.

ComposableCow drops 10,659 to 10,595 bytes.

getRootAndProof also loses its two function pointers, which existed only to inject murky's getRoot and getProof.

The ERC20 question

IERC20 in GPv2Order.Data was never OpenZeppelin's. It comes from cowprotocol through IConditionalOrder and BaseConditionalOrder, so handler Data structs were already consistent with the settlement types. Nothing about that changes here, and there is no mismatch to introduce.

The one place OpenZeppelin's IERC20 did leak in was test/vendored/GPv2TradeEncoder.sol, which now takes cowprotocol's like everything else.

Test mocks

Moved too, so this is a removal rather than a partial one. MockERC20 is rebuilt on solady's ERC20, which declares name and symbol abstract rather than storing them in the base, so the mock holds them now.

Dependencies after this

Four submodules: forge-std, cowprotocol, safe, solady. OpenZeppelin is gone directly and transitively.

Verification

172 tests pass, forge fmt --check clean, and rg 'openzeppelin|murky' src/ test/ script/ returns nothing.

Note for anyone touching this branch

jj does not record submodule gitlink changes, and this commit adds one (lib/solady) and removes two (lib/@openzeppelin, lib/murky). The tree was assembled with git plumbing, so jj git push here will undo both. Use git push.